Well, when I've used the term *tryhard* I usually referenced players which are taking the game way too seriously and act like their life depends on it.
That is a completely opinion based thing though, as I am playing games like DotA entirely for fun. If I meet a player in a public, unranked queue that wants to try out advanced tactics he has seen watching pro matches, I tend to think "what a tryhard", while others who take the game more serious than I do would appreciate that motivation the player is showing.

To summarize: *tryhards* are players who take the game more serious than I (or someone else) do. Obviously that is opinion based and I'd count it as an insult, therefore I am not using the term *tryhard* in general.