The smallest item, bit is an abreviation for 'binary digit'. They are the 1's and 0's often referred to when talking about computers. A solitary bit isn't particularly useful but put together in a series they can represent numbers using a binary system. Everything a computer does is done by performing calculations on binary numbers.
If you have ever seen morse code used, the bit is equal to a single flash of a light, or beep in telegraph morse.
Eight bits make a byte and this is the smallest collection of bits a computer can work with. A byte can represent any number from 0 to 255. Bytes represent letters, numbers, and all other symbols such as a comma, full stop, space etc. The format followed is a very old system (in computer terms) known as ASCII (American Standard Code for Information Interchange).
In morse code 3 light flashes equals 1 letter (dot dot dot, dash dash dash, dot dot dot for SOS is the one familiar to most of us). Morse is thus a 3 digit code. Computer code is an 8 digit code.
Just as with normal speech, single letters are not much use unless put together as words, bytes need to be strung together to for anything meaningful.
Due the numbers involved the smallest practical measurement of computer data storage is a kilobyte or a thousand bytes or characters (technically 1,024 to be accurate in computer terms as computers work in binary).
The term kilobyte is usually abreviated to Kb or KB or sometimes just 'k'.
Again due the mass of data Kb soon becomes meaningless and the terms megabyte (Mb)(million bytes) and gigabyte (Gb) (thousand million bytes) have now become quite common.
Just to confuse you even further, when referring to data transfer (for example when discussing modem or braodband speeds), this is spoken of in terms of kilobits NOT kilobytes, hence a 56k modem transfers data at a rate of 56,000 bits per second; and this time the kilo refers to 1,000 not 1,024
Speed (of processors or refresh rates of computer screens) is measured in Hertz, abreviated to Hz, and means one cycle of oscillation. Again due to the figures involved the term Hz is usually linked to K (1,000), M (1,000,000) or G (1,000,000,000)
A computer processor works to the tick of an internal clock and typically performs one calculation on every tick. How often the clock ticks (the clock speed) determines the speed of the computer and that speed is measured in MHz. Difficult as it may be to imagine something happening 1,000,000 times per second this is nothing in terms of computer speed.
To say that "how often the clock ticks determines the speed of the computer" is not technically acccurate however as all processors perform complex calculations by carrying out several sums in succession. Some processors are more efficient that others which means they take fewer ticks to perform the same complex calculation. The bottom line therefore is that it is possible for one procesor to be quicker than another even though it has a lower clock speed!