The first one is T$ and you can get this by playing various tournaments (Double Shootouts, Rebuy multi table tournaments, Freeze outs, and many more.  All of the tournaments can be found under the Tourney or Sit and Go tab, and then under Satellite sub-tab).  When you win one of these tournaments you MUST! un-register your seat by finding the tournament in which you won entry to and going to the lobby and un-registering via bottom left button. 

    The second type of currency is W$.  W$ can be earned in the same style tournaments as T$, but are found under the Events tab (WSOP, WPT, EPT,WCOOP, and various other special tournaments).  The value of W$ is much lower since there are much less ways of using W$.

    There are various reasons of why you would either want to sell T$ or W$.  The most common reason is to receive CASH if you cannot attend the event you won entry to.  The reason you would want to purchase W$ is if you plan on playing a W$ event or satellite.