What is a reaping?

A reaping is an annual event that takes place in every district before each Hunger Games where the tributes of the upcoming Games are chosen. Each district's escort randomly chooses the name of one male and one female tribute from two separate glass balls. Those picked are then the official tributes for the upcoming Games. To be qualified for a reaping, one must be 12 years old.

Once of age, a potential-tribute's name is entered into the reaping one time. The entries are cumulative, so when a possible tribute is thirteen, their name is added one more time. This continues until age 18, when a potential-tribute's name is entered 7 times, or more if they apply for tesserae.
Each age group is contained in roped-off areas in the town square, waiting for someone to be picked. The reapings take place at different times in different districts, so that Capitol citizens can watch them all live on television. On 74th Hunger Games, escort of District 12, Effie Trinket picked Primrose Everdeen and Peeta Mellark.

If someone so chooses, they may volunteer to take the place of a tribute who is selected. This is often done by Career Tributes who spend their lives training to compete in the Hunger Games. It is sometimes done for other reasons though, such as when Katniss Everdeen volunteered to protect her reaped sister, Prim, but this is not done nearly as often as Careers volunteering.
The volunteering process is simple in most districts, especially the later numbered ones. But in earlier numbered districts, such as the Career districts, (1,2and 4) volunteering is a more complicated process since participating in the Games is seen as an honor and victors are highly respected.