Medics can heal team members and poison the enemies. My advice for new players is to test all the classes until you get a feel for each, and find which ones you like the best and how to take advantages of the strengths of that class. You may find yourself to be a good spy, so you take advantage of being able to disguise yourself as a member of the opposite team, and use this to stab your enemies in the back. Due to all the player options, you can do so much experimentation, which aids in the prevention of getting sick of this game. I was still learning new tricks weeks after I started playing.
Teamwork is a key.
If you want to be successful, you have to work together. After sneaking in, one team mate causes a diversion, while another takes advantages of his spying abilities to grenade the sentry guns, while you slip in and grab the flag. Also, it's nice to have people watch your back. TFC is highly addictive, and with new maps added with every update, you don't have to worry about this getting old. If you like multiplayer first-person shooters like Doom and Quake, you will LOVE Team Fortress.