Bit hard to explain in great detail in a short post but it basically base64 is text that represents something such as a string, file, filestream etc.
The base64 text can then be sent via http, email etc. If you open a .eml (email file from outlook express) in something like notepad++ that has a file attached, you will see a long column of what appears to be random letters and numbers at the bottom. That is a base64 encoded file.
At the other end, it can be decoded to restore the original content.
My helpful sig has gone because a mod below the administrator gave me an infraction - despite the administrator personally agreeing to it.
If you need any php tips or tricks you can PM me.