TFSAs come in many forms, from simple savings accounts to GICs and other investments. You can hold almost any type of investment in your TFSA that you can hold in an RRSP, including mutual funds, ETFs, bonds, publicly-traded securities and even some private company shares.
You can hold foreign funds and securities in a TFSA as well. However, these will be converted to Canadian currency on the transaction, so be sure you understand current exchange rates to avoid over-contributing to your TFSA. Some foreign dividend income earned in a TFSA can also be subject to foreign withholding tax so keep that in mind when making these kinds of investments.
Finally, capital losses are not eligible for a tax write-off as they are in other circumstances, so if you lose money on your TFSA investments, you cannot recoup part of the loss with tax savings. Be sure to invest only to the level of risk you can currently tolerate.