Reinforced Concrete: You place forms, and the reinforcing steel and then pour it in the field.
Prestressed Concrete: These members are produced in factories under controlled environments. The steel used here is high strength steel, which is stronger than ordinary rebar. You place them as designed by the engineer, and typically towards the bottom of your beam section. After that with jacking equipment you pull them to a certain stress level. Then you pour concrete over it, and fill your cross section to make up your beam. After your concrete hardens, it can now hold the steel inside as they are bonded all over the length. So now you can release the jacking equipment at the end and cut ends of the steel. Now the stress of the steel is being taken by the concrete. Now the steel, forces the concrete at the bottom (as the steel itself is at the bottom) to compress. Due to this, the bottom of the beam is now under compression, and the the top has the corresponding tension, which means the beam deflects a little towards up. Why do you want this? Because when you take this beam to the field, and put load to it from the top, that load must first be at a certain level, just to overcome this upward curve, to make it flat, and only then, if it gets higher, it will sag the beam as usual. This means, until that load makes the beam straight, you just obtained an extra load carrying capacity. This is the purpose of prestressed concrete. it is very commonly used in certain slabs or beam type bridges.