Bit: Computers deal with binary digits or bits for short. A bit can be 0 or 1, equivalent or off or on.
Byte: One byte is eight binary digits, such as 1111001.
Kilobyte (KB): The smallest file stored on a smartphone, tablet, or PC is typically four kilobytes (4KB) in size. A kilobyte is 1024 bytes. Therefore 1KB is the same as 1024 x 8 = 8192 binary digits.
Megabyte (MB): 1024KB equals one megabyte (MB).
Gigabyte (GB): There are 1024MB in one gigabyte.
Terabyte (TB): There are 1024GB in one terabyte (TB)
Why your hard drive has a lower capacity than advertised
Hard drive manufacturers have long eschewed this system in favor of rounding down to make things easier (and also provide less storage space).
This means that 1000 bytes = 1 kilobyte and 1000 kilobytes = 1MB. Again, 1000MB = 1GB and 1000GB = 1TB.
Windows, however, sticks to the 1024 rule, which means it sees a 250GB hard drive as 232GB, and a 1TB drive as 931GB.
This explains why hard drives appear to have a lower capacity than advertised.
A 1TB hard drive has the capacity to store 1,000,000,000,000 bytes. Divide this by 1024 and you get 976,562,500KB. Divide by 1024 again and you get 953,674.3MB. Finally, divide by 1024 to get gigabytes and you end up with 931.32GB.