This is the symbol table. Supported symbols are listed here (alphabetically).
Symbols can also be trained here. Just pick a symbol you sometimes need but tend to forget and click it. A canvas will open for your training input.
If there are symbols missing
be sure to drop me a line (firstname.lastname@example.org)*. If you have more specific LaTeX questions though, you will have more success contacting your TeX user group or try the excellent TeX Stackexchange.