The REVTEX macro package allows two methods for numbering equations: you can assign your own equation numbers or you can allow REVTEX to number for you.
Use the command \eqnum{thenumber} to number your own equations. You can also use this command to produce a specific equation number not normally obtainable; (1'), for example.
For automatically numbered single-line and multiline equations, use the equation and eqnarray environments. You can use the \[, \] commands and the eqnarray* environment for unnumbered single-line and multiline equations, respectively. The command \nonumber will suppress the numbering on a single line of an eqnarray.
If you wish a series of equations to be a lettered sequence, e.g., (1a), (1b), and (1c), just include the equations or eqnarray within the mathletters environment.
Finally, to have REVTEX number equations by section, use the eqsecnum style option.
See Sample Compuscript A to see examples of how all this works.