What's the difference between a word and byte?
Byte: Today, a byte is almost always 8 bit. However, that wasn't always the case and there's no "standard" or something that dictates this. Since 8 bits is a convenient number to work with it became the de facto standard.
Word: The natural size with which a processor is handling data (the register size). The most common word sizes encountered today are 8, 16, 32 and 64 bits, but other sizes are possible. For examples, there were a few 36 bit machines, or even 12 bit machines.
The byte is the smallest addressable unit for a CPU. If you want to set/clear single bits, you first need to fetch the corresponding byte from memory, mess with the bits and then write the byte back to memory.
The word by contrast is biggest chunk of bits with which a processor can do processing (like addition and subtraction) at a time. That definition is a bit fuzzy, as some processor might have different word sizes for different tasks (integer vs. floating point processing for example). The word size is what the majority of operations work with.
There are also a few processors who have a different pointer size: for example, the 8086 is a 16-bit processor which means its registers are 16 bit wide. But its pointers (addresses) are 20 bit wide and were calculated by combining two 16 bit registers in a certain way.
What I don't understand is what's the point of having a byte? Why not say 8 bits?
Apart from the technical point that a byte isn't necessarily 8 bits, the reasons for having a term is simple human nature:
economy of effort (aka laziness) - it is easier to say "byte" rather than "eight bits"
tribalism - groups of people like to use jargon / a private language to set them apart from others.
Just go with the flow. You are not going to change 50+ years of accumulated IT terminology and cultural baggage by complaining about it.
FWIW - the correct term to use when you mean "8 bits independent of the hardware architecture" is "octet".
BYTE
I am trying to answer this question from C++ perspective.
The C++ standard defines ‘byte’ as “Addressable unit of data large enough to hold any member of the basic character set of the execution environment.”
What this means is that the byte consists of at least enough adjacent bits to accommodate the basic character set for the implementation. That is, the number of possible values must equal or exceed the number of distinct characters. In the United States, the basic character sets are usually the ASCII and EBCDIC sets, each of which can be accommodated by 8 bits. Hence it is guaranteed that a byte will have at least 8 bits.
In other words, a byte is the amount of memory required to store a single character.
If you want to verify ‘number of bits’ in your C++ implementation, check the file ‘limits.h’. It should have an entry like below.
#define CHAR_BIT 8 /* number of bits in a char */
WORD
A Word is defined as specific number of bits which can be processed together (i.e. in one attempt) by the machine/system. Alternatively, we can say that Word defines the amount of data that can be transferred between CPU and RAM in a single operation.
The hardware registers in a computer machine are word sized. The Word size also defines the largest possible memory address (each memory address points to a byte sized memory).
Note – In C++ programs, the memory addresses points to a byte of memory and not to a word.