Why is a Byte 8 bits?: The 8-bit byte was not always standard. Computers used many different "byte" sizes over the course of history, depending on hardware and how addressable memory worked. However, much of the early computing world relied on representing data and computer instructions encoded in ASCII text where every character is 8 bits. Thus, 8-bits was such a common chunk-size for representing information that it stuck and they gave it its own name - byte.

There are various accounts about why it was called a “byte” but most point to early days at IBM where “bite” was used to to refer to groups of 8-bits that a computer was processing, as in it could “bite” off 8 bits at time. The spelling was changed to “byte” to avoid confusion with “bit”.

Bytes became the fundamental unit with which we measure the “size” of data on computers, and in fact, today most computers only let you save data as combinations of whole bytes; even if you only want to store 1 bit of information, you have to use a whole byte to do it.

NOTE: A 5-byte file is so small that some computers won't allocate a chunk of memory that small. For example you might see something like this:

Which indicates that even though the file is 5 bytes, it's taking up 4 Kilobytes of memory on your computer.

There are some discrepancies in common usage of the kilo, mega, giga prefixes.

From the Stanford CS 101 website:

It's convenient within the computer to organize things in groups of powers of 2. For example, 2¹⁰ is 1024, and so a program might group 1024 items together, as a sort of "round" number of things within the computer. The term "kilobyte" above refers to this group size of 1024 things. However, people also group things by thousands -- 1 thousand or 1 million items.

There's this problem with the word "megabyte" .. does it mean 1024 * 1024 bytes, i.e. 2²⁰ which is 1,048,576, or does it mean exactly 1 million, 1000 * 1000. It's just a 5% difference, but marketers tend to prefer the 1 million, interpretation, since it makes their hard drives etc. appear to hold a little bit more. In an attempt to fix this, the terms "kibibyte" "mebibyte" "gibibyte" "tebibyte" have been introduced to specifically mean the 1024 based units (see wikipedia kibibyte article). These terms do not seem to have caught on very strongly thus far.

If nothing else, remember that terms like "megabyte" have this little wiggle room in them between the 1024 and 1000 based meanings. For purposes of CS Principles the distinction is not important - "about a million bytes" is a fine, close-enough interpretation for "megabyte".