Hi
Research & Development will eventually get your team/car better brakes, aerodynamics, cooling ..... that sort of thing. Usually the number 1 driver in the team will get the updates before the number 2 driver.
So your car will get better throughout the racing season.
In the 2013 season, each car was allowed 5 gearboxes and 10 engines. Geaboxes are not simulated in the game, but the engines are. So those 10 boxes are the engines that have to last you the whole year.
So each engine has to last you about 2 races. You can save engines for tracks that stress the engine, and do you practice session on old engines. Bear in mind that changing an engine will cost you the rest of the session you are in at the time. This wasn't so bad in previous games, as there used to 3 practice sessions (like real life), but CM cut it to one session for this game, much to the annoyance of many people.
So by all means practice using your old engines, and change it to a new one (if you want) just before the session ends, so you don't lose much track time.
That's more or less what's happening ........