Computing data is referred to in a variety of different units, most derived from the byte unit.
A bit is a state of either "on" or "off". It is the smallest possible unit of data on any computer.
The term byte represents an ordered collection of bits, with each bit representing a binary value of either 0 or 1. In most modern hardware, the defined size of a byte is eight bits, hence usually there are 2^8 (or 256) distinct values in one byte.
In the Pokémon games, the smallest groups of identifiers can only be read by the console in single 8-bit bytes. This explains, for example, the reason why there are 105 glitch Pokémon in Generation I: 2^8 is 256, and 256 - 151 = 105.
Bytes are also sometimes read as arrays of bits, in other words an ordered list of eight "ons" and "offs", for instance when storing badges obtained in Pokémon Red, Blue, and Yellow.
The size of a word depends on the type of hardware. On 32-bit processors, such as that of the Game Boy Advance, the size of a word is four bytes. A unit of two bytes in this architecture is referred to as a halfword, and a unit of eight a double word or dword.