The tower controller is probably one of the simplest job of all the ATC sectors. These are the things the Tower will do:
Give takeoff clearances
Give landing clearances
Let aircraft cross active runways
And sometimes let aircrafts fly throught his airspace.
Now keep in mind, the airspace of the tower controller is very small. As soon as a commercial jet aircraft has lifted he will tell him to contact departure. The airspace will normally go up to 2000 feet AGL (Above ground level). And it will stretch out to around 10 nM from the airport. Above and beyond that is the airspace of the approach and departure.
When an aircraft has taxied to the holding short position, he will contact the tower and ask for takeoff.
Upon takeoff the tower most include these things when clearing an aircraft for departure
1. The wind (diretions and speed)
2. The runway
3. If required; Causion wake turbulence
4. Clear for takeoff
Here are a few examples of tower scenarios when giving takeoff clearances.
Scandinavian 5245 holding short of runway 26 on taxiway A, requesting takeoff
ATC: SAS5245 wind calm, runway 26 clear for takeoff!