tf2 keys?

yes

Yes. You use them to open crates. You have to buy them though.

TF2 keys are keys that open up crates you can get as drops in the game. You can get them from the TF2 store, or from the market or from trading items with people.

just a item from tf2 that people use as currency when trading games as the only way to give money is through paypal and other sites like that which a lot of people don't want to use.

yes

And they are often used as "money".

Atm a tf2 key is worth 1,50$-1,70$, so you can trade for example 3 keys for a 5$ game. But please do that on steamtrades.com and not here^^

a key is worth 5 refined metal, 1 reclaimed metal and 2 scraps in trade atm. they are worth $2.20 aprox in the community market and 2.49 in the mann co store

They are used to open crates (which randomly drops when playing) from which you can get random items.