The memory of a computer can be thought of as a series of boxes, each containing 8 bits (1 byte), and each with its own unique address, counting from zero upwards.
The memory capacity of a computer is measured in thousand-byte units called kilobytes, megabytes or gigabytes.
These measures can be abbreviated to Kb, Mb and Gb.
These are all powers of 2; thus although 1Kb is often thought of as being 1,000 bytes, it is actually 1024 bytes.
So:
2¹⁰ bytes = 1024 = 1Kb
2²⁰ bytes = 1024 x 1024 = 1Mb
2³⁰ bytes = 1024 x 1024 x 1024 = 1Gb
2⁴⁰ bytes = 1024 x 1024 x 1024 x 1024 = 1Tb