Little more detail.

Bit: A Bit is the smallest unit of data that a computer uses. It can be used to represent two states of information, such as Yes or No.

Byte: A Byte is equal to 8 Bits. A Byte can represent 256 states of information, for example, numbers or a combination of numbers and letters. 1 Byte could be equal to one character. 10 Bytes could be equal to a word. 100 Bytes would equal an average sentence.

Kilobyte: A Kilobyte is approximately 1,000 Bytes, actually 1,024 Bytes depending on which definition is used. 1 Kilobyte would be equal to this paragraph you are reading, whereas 100 Kilobytes would equal an entire page.

Megabyte: A Megabyte is approximately 1,000 Kilobytes. In the early days of computing, a Megabyte was considered to be a large amount of data. These days with a 500 Gigabyte hard drive on a computer being common, a Megabyte doesn't seem like much anymore. One of those old 3-1/2 inch floppy disks can hold 1.44 Megabytes or the equivalent of a small book. 100 Megabytes might hold a couple volumes of Encyclopedias. 600 Megabytes is about the amount of data that will fit on a CD-ROM disk.

Gigabyte: A Gigabyte is approximately 1,000 Megabytes. A Gigabyte is still a very common term used these days when referring to disk space or drive storage. 1 Gigabyte of data is almost twice the amount of data that a CD-ROM can hold. But it's about one thousand times the capacity of a 3-1/2 floppy disk. 1 Gigabyte could hold the contents of about 10 yards of books on a shelf. 100 Gigabytes could hold the entire library floor of academic journals.

Terabyte: A Terabyte is approximately one trillion bytes, or 1,000 Gigabytes. There was a time that I never thought I would see a 1 Terabyte hard drive, now one and two terabyte drives are the normal specs for many new computers. To put it in some perspective, a Terabyte could hold about 3.6 million 300 Kilobyte images or maybe about 300 hours of good quality video. A Terabyte could hold 1,000 copies of the Encyclopedia Britannica. Ten Terabytes could hold the printed collection of the Library of Congress. That's a lot of data.

Petabyte: A Petabyte is approximately 1,000 Terabytes or one million Gigabytes. It's hard to visualize what a Petabyte could hold. 1 Petabyte could hold approximately 20 million 4-door filing cabinets full of text. It could hold 500 billion pages of standard printed text. It would take about 500 million floppy disks to store the same amount of data.