AMSThm package. Example:

 usepackage [] amsthm

Create a new theorem environment type. Arguments are environment name (this will be the argument when using the theorem function) and the displayed title.

For example:

 newtheorem "prop" "Proposition"

 theorem "prop" "This is it."

Use a environment created by newtheorem.

The proof environment. The first optional argument is used to put a custom title to the proof.

Insert the QED symbol.

Different styles for theorems.

Set the theorem style. Call this function in the preamble.