I don't remember now if certain environments I mention below come from the vanilla LaTeX or from the amsmath package, so I'll assume that amsmath is included.
Unstarred environments for display math, such as align and gather, add numbers to equations. Starred environments like align* and gather* don't. There is an unstarred environment for a single equation: equation. As far as I understand, equation* is equivalent to \[ ... \]. You can add a tag where none is provided by default or replace a tag using the \tag command. Its argument is typeset in text mode and is surrounded by parentheses unless you use \tag*. You can also suppress tags with \notag.
I recommend reading the amsmath user guide or a chapter about math in some good book on LaTeX such as The LaTeX Companion. It's well worth the time.