There are many different units of memory (or RAM), and I will list them here, starting from the smallest:
Byte: A byte is the smallest of all the units of measuring. It is "hard" to get a file or folder that is only one byte, unless you have an empty text file in a folder, you may get one byte.
KB (Kilobyte): This is one thousand bytes, and is commonly found with short text files or low quality pictures.
MB (Megabyte): This is one thousand KBs, and is very common, and can be found in medium-quality and high-quality pictures, folders, files, and lots of other types.
GB (Gigabyte): This is one thousand MBs, and is just as common as MBs. It is very rare that you would find a text file with a GB worth of text, and it is common with games or programs.
TB (Terabyte): This is one thousand gigabytes, and I don't think there is any file or folder or program that takes up a TB. It is mostly used for hard drives in a computer.