Data is stored in bytes on the computer and networks. A byte is normally the amount o space it takes to store one character has been the historical measurement. However as we expanded computer code to include all the different single characters in the world it now takes two bytes normally to store one character as we know them. A single character could be a single digit number, one letter or a special character like @, $ or others.
The amount of space available to store the data in on disks, CD/DVDs, flash/thumb drives is measured in billions or trillions of byres today. A drive with 1 terabyte of space has 1 trillion byes or 10 raised to the power of 12 of space or can store 500 billion characters.
Pictures and videos take a lot of space. Most phones and cameras pictures each take from 5-20 megabytes to store. A megabyte is a million bytes.
Gigabytes are billions of bytes and kilobytes are thousands of bytes. When the first PCs came out about 198-0 they had 16kilobyes of memory and the first hard disks for PCs in about 1983 were 5 megabyes in size.
The next size we will see is petabytes which are 1000 terabytes. Mainframe/super computers and data warehouses are sometimes using that now. You also see exabytes being mentioned some which are 1000 petabytes.