This mission has a crew of only four. Its primary purpose is to deliver supplies and parts to the International Space Station. From two different articles at the NASA site:
Space shuttle Atlantis lifted off July 8 on the final flight of the shuttle program, STS-135, a 12-day mission to the International Space Station. Atlantis carries a crew of four and the Raffaello multipurpose logistics module containing supplies and spare parts for the space station. The STS-135 astronauts are: Commander Chris Ferguson, Pilot Doug Hurley, and Mission Specialists Sandy Magnus and Rex Walheim.Usually, there are three other mission specialists on a Shuttle flight, but it seems this isn't really a normal flight.
The mission also is flying a system to investigate the potential for robotically refueling existing spacecraft and returning a failed ammonia pump module to help NASA better understand the failure mechanism and improve pump designs for future systems.
The Shuttle program is thirty years old. In that time, it has completed 134 missions. Two missions ended disastrously, with the loss of fourteen crew members. That is a rather high cost. But, in comparison to the Apollo program, the Shuttle was almost the routine transportation system it was meant to be. One Apollo mission caused the deaths of three crew without even leaving the ground; an equipment fire killed all three astronauts in the Apollo One mission. Apollo Thirteen nearly killed its crew thanks to an oxygen tank explosion. Out of twelve manned missions, one killed its crew, and one almost did. In that light, the Space Shuttle, which while it did not venture beyond Earth orbit nevertheless stayed in space roughly as long on each mission, had roughly one tenth the serious accident rate.
In the case of the Shuttle, each disaster was due to a lax attitude among the leadership of NASA, who took for granted that a one out of four chance of a rocket exploding meant that it wouldn't explode on their watch. Perhaps the ultimate tragedy of the second Shuttle disaster was that it was caused by the same foolish management attitude that caused the first one, and the same unwillingness to listen to the concerns of the astronauts and engineers.
If you're wondering what comes after, all I can say is so am I. As I mentioned earlier, it's extremely unlikely that NASA will have another manned spaceflight program within five years. That makes it nearly impossible to hold onto the people who now know how to operate such a program. That the last two presidential administrations have allowed this to happen is yet another example of what lousy leadership we have in DC these days. It's possible that the "commercial" launch programs, where NASA pays commercial aerospace companies to develop spacecraft, will take up some of the slack, but that's at least three years away, and there's no guarantee that that program will come to fruition. If there's no financial motivation to develop it, a commercial program can be canceled just as quickly as a government program, if not more quickly.
The success of the Shuttle program came from the abilities and determination of the flight crew, technical staff, and operational staff who designed the spacecraft, then flew and operated it. Their leaders, both at NASA and in DC, can be thanked for its failures and its end with no viable replacement.