A "Top" is one who takes the *giver* (rather than the receiver) role in SM/fetish sex. The Top is the person who does the bondage, gives the whipping. No Power Relationship is implied. Put another way, all Dominants, sadists and Masters or Mistresses are Tops because they all are the ones who *give* sensation; but not all Tops are Dominants or sadists or Masters/Mistresses, because they do not necessarily act as authorities or include pain-play in their repertoires. For this reason, it is common for a submissive to 'top' their submissive friends in a scene. They can give a friend a complete experience, as long as the friend doesn't need power exchange to have a good time.
A "bottom" is the receiving partner in an SM/fetish dynamic, and is the counterpart to a "Top". More often than not, people identify as bottoms to let potential partners know that they are not seeking a Power Exchange but either a hedonistic experience or role play.
There is a reason why clinicians joined the prefix 'sado' to 'masochism': the vast majority of kinky people have fantasies on both sides. At various times in their lives, and for various reasons, Dominants/Tops/sadists may elect to switch and act out their submissive/bottom fantasies. Some of them switch with their partners, granting a submissive or bottom temporary license to take control. Others prefer to keep power roles intact and sub to another Dominant instead. This is where terms like "Top" and "bottom" become especially helpful. A Dominant may have no interest in genuinely giving up control but may find an occassional foray into submissive fantasies to be fun. By offering to "bottom" rather than to "submit", the Dominant essentially protects His or Her basic identy. There are also Dominants who want real submissive experiences, but only periodically. One caveat: If a Domninant seeks out submissive or bottom experiences more than Dominant experiences, then what you have is either a Dominant evolving in a new direction or a submissive in denial.