The driving force comes from the weights, or spring whichever the clock has.
The escapement/pendulum combination is what controls the clock speed, i.e. what keeps time. The period of the pendulum swing depends only on the length of the pendulum provided it is only swinging around 5 degrees or so.
Each swing of the pendulum allows the escapement to move one (or two sometimes I think) teeth. So the pendulum swing counts out equal units of time; they may be whole seconds or parts of seconds depending on the length of the pendulum.
Each time the pendulum releases the escapement it receives a little kick to keep it swinging at the same amplitude; this kick compensates for the loss of kinetic energy in the pendulum due to friction. Without the kick from the escapement the pendulum would eventually stop; with too much kick the pendulum swings wider and wider and does not keep a constant period if it gets too wide. This is the tricky part of escapement design.
All the other gears just give the correct ratios between the rate of rotation of the escapement wheel and the clock hands.