If you have not voted early in the Republican or Democratic primary, July 31 is your last chance. It's a run-off election, to determine the nominees of each party.
You are allowed to vote in the election even if you did not vote in May, but you are not allowed to vote in the run-off of one party after voting in the May primary of another.
The most publicized race is for the U.S. Senate, between Ted Cruz and Lt. Gov. David Dewhurst, but there are other contests around the state as well, including a few for the Legislature.
If Mr. Cruz wins, Mr. Dewhurst will (absent any unforeseen circumstances) remain as Lt. Gov., a very important position in Texas regarding the Legislature. The Lt. Gov. presides over the Senate, making committee appointments and setting the agenda for legislation.
Many times the outcome of the primary determines who will take office, since districts are carefully drawn to maximize party strength and incumbency.
Here's a link to determine "Who Represents Me?" You can simply type in your address and zip code.
Here is a very helpful site from the Texas Secretary of State, which contains information for voters about ballot locations, registration, and the mechanics of voting in our state.