A lot of useful information is in the mode line, the line at the bottom of each Emacs window. Normally, the mode line looks something like this:
-CS:CH BUF (MAJOR MINOR)--LINE--POS------
CH contains two stars if the buffer has been changed since the last save, otherwise two hyphens. If the buffer is read-only, it will show “%%” if it has not been changed and “%*” if it has been.
Inside the parentheses is the current major mode, sometimes with more information about the major mode's state. This is followed by the names of any minor modes in effect, such as abbrev mode (which will convert abbreviations you define to their expanded strings) or auto-fill mode, discussed elsewhere in this article.
BUF, the name of the buffer, is obvious. It is usually (but not always) the name of the file. To the right, “LINE” is an “L” followed by the current line number (if line number mode is on and it usually is). To the right of that, an approximate position in the file is indicated by a percentage or All, Top or Bot, as appropriate.
Of course, you can customize the mode line. Many major and minor modes customize it, and users can add their own modifications. For example, you can see the current time and date in most of the screen captures that accompany this article. That is my own customization.