The basic modes are:
Normal mode - used for editor commands. This is also the default mode, unless the insertmode option is specified. Keystrokes produce actions not typed characters. Hit ESC in another mode to retreat back to Normal mode.
Insert mode - similar to editing in most modern editors. In insert mode, buffers can be modified with the text inserted.
Command-line or Cmdline mode or LastLine Mode - supports a single line input at the bottom of the Vim window. Normal commands (beginning with :), and some other specific letters corresponding to different actions (including pattern search and the filter command) activate this mode. Used for overall operations: saving and exiting, loading files, searching and replacing.
Visual mode - similar to normal mode, but used to highlight areas of text. Normal commands are run on the highlighted area, which for an instance can be used to move or edit a selection.
Select mode - works similarly to visual mode. However, if a printable character, carriage return, or newline (or line feed) is entered, Vim inserts the character, and starts insert mode.
Ex mode - similarly to Cmdline mode, it takes a single line input at the bottom of the window. However, in Cmdline mode, entering a command exits the mode when the command is executed. Entering a command in Ex mode doesn't cause the mode to change.