Players
A team is the virtual football team one owns and manages in Hattrick. It consists of a number of virtual football players, usually between 20 and 30 (maximum 50). The manager chooses a formation with 11 of these players for each game, plus 5 substitutes.
If the 50-players limit is exceeded, Training is lost for that week. Please take note that the Coach is considered a player, as he may well play (and often does!)
Each player has an individual name, and is made up of assorted skills, personality, form, experience and specialties. He'll also have an ideal position, but you'll need to work that out through experience!
Skills
Each player has 8 basic skills, which can be improved through training, and decline with age. These skills are Stamina, Playmaking, Wing, Scoring, Goalkeeping, Passing. Defending and Set Pieces. Each of these skills has a a value between disastrous and divine.

Form
Form is one of the most important characteristics for any player, determining how well he is able to use his skills. You'll note that a good player who is out of form will often be outperformed by a not so good player at the top of his game.