There was a major Team Fortress 2 tournament this weekend and I watched some on Twitch just to see what it was like. Here's some of what I've gleaned:
This particular tournament, the ESEA Invite S17 LAN Finals, is 6s tf2: each team has one medic, one demoman, and four other players, usually two soldiers and two scouts. These teams fight over control points in a variety of arenas.
Scouts are fast, can double-jump, and have hitscan weapons (damage happens instantly when you click on an enemy), and are thus good at taking out individual targets and, of course, scouting.
Soldiers have rocket launchers that do splash damage (damage spread out over a small area). Soldiers can use their rocket launchers to jump long distances, making them very mobile. One soldier serves as a 'roamer', jumping around, flanking positions, and trying to kill the enemy medic. The other soldier is a pocket, staying close to the friendly medic, protecting them, and pushing forward as the vanguard of the team.
The demoman's job is to control areas-- the demo's sticky bombs and explosive pipes can keep enemies from advancing, especially in narrow doorways and alleys. If a medic is using the kritzkrieg (an offensive medigun that increases a teammate's damage), it will usually be the demoman that is given the extra damage-- a kritzed demo is formidable indeed.
The medic is the team healer and often the team's strategist, calling out plays and enemy positions. Medics also can use their mediguns to make teammates invincible or kritzed. This is a hugely important role, and killing the enemy medic is of highest priority to each team.
As with all sports, seeing star performers at work is half the fun of watching. The winning team of this tournament, Froyotech, is led by b4nny, a 21-year-old often cited as the best tf2 player in the world. Clockw0rk, also part of Froyotech, is called the best scout in North America and took out four players in 30 seconds in one of the grand finals matches this weekend.
There's a lot more that could be said, but the biggest takeaway I have from this tournament is that it's hard to see much difference between watching esports and watching real sports.