MEDIC
medic work alone - revives & teamplay. For example, most of the better Aimers are medics, but that doesn't necessarily mean that they are great medics for their team, just that they are incredible aim who play medic for the health. Keeping your team alive and reviving at crucial moments is the key. With a medic there is always a choice - get the revive or go for the kill. Always the better medics get the revive then the kill .
ENGINEER
Objective play is crucial - making sure the tank is built and escorted, CP is built, gates are blown, etc. As the years have gone by, the engineer role has become more aggressive and really has taken over the role of field-ops as a spam orientated damage dealer, so the ability of these players to contribute towards the damage and frags, via aim, landmines & grenades should also be considered.
I've taken this off crossfire.nu - just so some new players can have an idea on what exactly the roles are at a competitive level..
Moments like these are expected in et: