The text you are editing in Emacs resides in an object called a buffer. Each time you visit a file, a buffer is used to hold the file's text. Each time you invoke Dired, a buffer is used to hold the directory listing. If you send a message with C-x m, a buffer is used to hold the text of the message. When you ask for a command's documentation, that appears in a buffer named *Help*.
Each buffer has a unique name, which can be of any length. When a buffer is displayed in a window, its name is shown in the mode line (see Mode Line). The distinction between upper and lower case matters in buffer names. Most buffers are made by visiting files, and their names are derived from the files' names; however, you can also create an empty buffer with any name you want. A newly started Emacs has several buffers, including one named *scratch*, which can be used for evaluating Lisp expressions and is not associated with any file (see Lisp Interaction).
At any time, one and only one buffer is selected; we call it the current buffer. We sometimes say that a command operates on “the buffer”; this really means that it operates on the current buffer. When there is only one Emacs window, the buffer displayed in that window is current. When there are multiple windows, the buffer displayed in the selected window is current. See Windows.
Aside from its textual contents, each buffer records several pieces of information, such as what file it is visiting (if any), whether it is modified, and what major mode and minor modes are in effect (see Modes). These are stored in buffer-local variables—variables that can have a different value in each buffer. See Locals.
A buffer's size cannot be larger than some maximum, which is defined by the largest buffer position representable by Emacs integers. This is because Emacs tracks buffer positions using that data type. For typical 64-bit machines, this maximum buffer size is 2^61 - 2 bytes, or about 2 EiB. For typical 32-bit machines, the maximum is usually 2^29 - 2 bytes, or about 512 MiB. Buffer sizes are also limited by the amount of memory in the system.