An individual that tries. Hard.
This is not in reference to those that are scholastically ambitious, or put effort into sports. No, no. This is a strong, biting description of someone that obviously thinks a whole lot about their public image - fashion, vernacular, taste, etc. Such a person tends to fit very neatly into a particular pigeon hole - typically, a subculture. A tryhard is pathetically hyper-aware and irredeemably lame in their awareness and internalizing of all things external and obvious.
Pretty much the only phrase used is uttered snidely, under the breath to one's cohorts: "What a tryhard" or, alternately, "whatta tryhard". In particularly outstanding cases, "oh my God, whatta tryhard."