The monster descriptions and statistics basically use the format of the 3rd Edition D&D rules, as described in the introduction of the Monster Manual and Monster Manual II. This is only a short summery of the elements, which are probably not much use without the books mentioned above and the organisation of the descriptive text is somewhat different.
Size and Type: is the creatures size category followed by its type and possibly subtype, as described in the MM. The types are Aberration (bizarre natural creatures), Animal (natural creatures existing on earth), beast (natural creatures not existing on earth), Construct (manufactured creatures), Dragons (big reptilian monsters), Elemental (from the elemental planes), Fey (mythic humanoid creatures), Giant (really big humanoids), Humanoid (human-like beings), Magical Beast (Beasts with magical abilities and more intelligence), Monstrous Humanoid (Humanoids with monstrous features), Ooze (literally living slime), Outsider (Things from other planes), Plants (which are little more alive than usual), Shapechanger (can change between several forms), Undead (once living beings) and Vermin (insects and other invertebrates).
Main Statistics
Hit Dice: The creature's hit dice and average
hit points. HD also double as the creature's level.
Initiative: The creature's initiative modifier.
Speed: The creature's speed, on ground first, followed
by any other type of movement the creature is capable of. Climbing and swimming
creatures can always take 10 on the according skill check, even if rushed or under
pressure. Flying and swimming creatures can take the run action while moving that
way. Flaying creatures can charge from the air and have a maneuverability class,
as described in the DMG and MM.
AC: The creature's armour class and what contributes to
it, followed by flat-footed and touch ACs.
Attacks: The creatures attack bonus and weapons. Attacks
joined by "and" can be used together in one round, attacks separated by an "or" not.
Creatures can attack with natural or manufactured weapons.
Damage: Natural weapons and any special damage in addition
to hit points it might do or the damage of its weapon.
Face/Reach: The space a creature occupies and its reach. If
its reach is larger than typical, the reason is given. See the introduction in the MM
or Chapter 8: Combat in the PHB for more information.
Special Attacks: Any special attacks the creature has, like
ability damage, breath weapon, energy drain, frightful presence or spell-like abilities.
Special Qualities: Any special qualities, like damage
reduction, regeneration, resistances, or special vulnerabilities. See the introduction
of the MM or Chapter 3: Running The Game in the DMG for more information.
Saves: The creature's saving throw modifiers.
Abilities: The creature's average ability scores. Some
creatures, like undead or constructs lack some abilities, because they are not
subject to it.
Skills: The creature's skills. If you increase a creature's
HD, the skills given in the entry are considered class skills, everything else
cross-class.
Feats: Creatures have the same feats as characters and some
more, which are described in sidebars in the MM and MM 2. Some creatured may have
feats marked as bonus feats with a (B). A creature does not need to fulfil the
prerequisite conditions to use these feats.
Secondary Statistics
Climate / Terrain: This is where the creatures is most
liekly found and naturally lives. Magic, slavery and other things can actually get
a creature pretty much anywhere, though it might resent being removed from its
natural environment.
Organisation: The typical group of creatures met. Groups
of natural beings often include a number of beings with none or very little combat
ability, children or workers for instance.
Challenge Rating: The level of party of at least four
fully rested characters that should be able to overcome a single member of the
described species without suffering casualties, unless the dice seriously turn
against them.
Treasure: The type of wealth and treasure owned by the
creature, referring to table 7-4: treasure in the DMG. Intelligent creatures will
use magical and other useful items. Creatures that do have lairs will keep most
treasure there and not carry it arounnd. Good luck finding and retieving it.
Intelligent creatures might guard their treasure with devious traps.
Alignemnt: The creatures alignemnt, as described in
Chapter 6: Description in the PHB. A creature that always has an alignment
does not really have a choice. Dragons, elementals and outsiders typically have
strong alignments like that. Creatures usually having an alignemnt have
a strong disposition to be of that alignment, but have a choice. Beings that
have an alignemnt "often" are as likely to have the given alignment as not,
depending how and where they grow up.
Advancement: The statistics given are for the weakest
and most common members of a species, either in HD or by character class, in which
case there is a section describing the creature's favourite class and other classes
typical for it to take at the end of the descriptive text.
Descriptive Text
Appearance: "How many tentacles does it have?" The first paragraph describes what the monster actually looks like. Since these entries generally do not contain images, this section is probably somewhat longer than those in the sources published in print, depending on how likely I think the average reader knows what I am referring to. A human with purple skin is probably easier to describe than a wierd aberration with blades, tentacles, mandibles and whatnot.
Mentality: describes the general mindset and mental conditions of this species, what drives them, what are they likely to think and why are they doing what they do.
Habits: Typical habits and mannerisms of the creature. What a group of player characters is most likely finding the creature doing when encountering it in its natural environment. Of course beings placed as sentries or part of armies are likely to act according to their task, unless they do it unwilingly and their superior is not looking.
Combat
Describes the creatures typical combat tactics, strategy and attacks it is most likely to use. How it opens battle, behaves in battle and if it is more likely to fight to death or flee when wounded.
In addition this section describes all of a creature's special attacks and qualities, in alphabetical order. Abilities marked with (Ex) are extraordinary and work at all times. Attacks or Quilities marked with (Su) are supernatural and cease functioning in antimagic fields, dead magic areas and other places where mystic powers fail. Spell-like abilies (Sp) work like spells without any visible components and a casting time of 1 action (unless using a feat to quicken it). Spell-like abilities can be resisted by SR.
Background Information
Society: "How does it live?" Most sentient beings cluster together and form a society, which can be as simple as family groups and as complex and intricate as anything humans made during their history on Earth. This section has to be somewhat general, for describing a complex society in full could fill entire tomes, and in case of some of the more interisting monsters this was actually done in the monstrous arcana books.
Origin: "Where does it come from?" A basic history and background on the creature. Construct and animated undead lack this section and get a description of the construction and / or spells necessary to create these beings at the the end of the entry.
Ecology: "How does it fit into the world?" Any natural creature interacts and influences its environment in some way and has a part in a world's ecosystem. What does it eat, what makes prey from it and does it have natural enemies? In addition many creatures have parts that are useful as components for spells or making magical items.
Construction or Chracters: Another section, with a headline, which describes charaters of the race, or spells and construction processes to make artificial beings and animated undead. Spell entries follow the rules from the PHB. Natural creatures only advancng by HD do not have this section.