What is Overwatch and what is the concept of the game?

My friends have been telling me to get this game that came out not too long ago, and I have been thinking of saving my money to buy some games for the summer and one of them is Overwatch. And I have been wondering before I buy it, what is it about and what is the whole concept/idea of the entire game?

Overwatch is an arena-based FPS that focuses on unique characters with set weapons/skills with an emphasis on teamplay. Think Unreal Tournament meets Team Fortress 2. You've got offensive characters like McCree the gunslinger, Reaper the wraith guy, Soldier: 76 the staple assault/rocket class, then there's defensive characters that do things like turrets or they specialize in defending points, then there's the support characters like the snipers, medics, flying characters, etc... and then there are the tanks, beefy characters that usually throw up shields or can absorb a lot of damage.

It looks like a pretty interesting game and I wanna try it out, but sadly my computer's not powerful enough.