Sport psychology can be pretty complicated, and sometimes it can seem a little out there.  Sometimes the techniques are specific to individual sports.  But basically, most sport psychology boils down to several key concepts that the psychologist-coach tries to get the athletes to focus on.  These concepts include:

Visualization
Having the athletes picture in their minds, the successful completion of some move, shot, or play.  Having the image in your head of what the right way to do it is, helps you accomplish it for real when the time comes.

Goal-setting
Coaches, teams, and individual athletes are encouraged to set realistic, short-term goals.  Meeting and exceeding these goals builds confidence and enables the setting of higher goals.  If the initial goal is too high, you'll get discouraged by not meeting it and will stop working toward anything; it's better to start small and build up.

Positive attitude
Perhaps the most important message of sport psychology is to think positively.  Identifying the kinds of negative thoughts that hold you back, and replacing them with a positive attitude toward your abilities, will allow you to play your best. 

Focus
Eliminating distracting thoughts and improving concentration is necessary for success.

Motivation
Setting personal standards to measure success.  Success and failure is more than just winning or losing the game or the race.  Sport psychology tries to help athletes measure themselves by their own effort, not by the outcome.  You're a better athlete (and a better person) if you tried your hardest but didn't win the game, than if you loafed but managed to win anyway. 

Team-building
Trying to improve communication and mutual encouragement and support among teammates, and improve the sense of team spirit and respect for the team.  With so much of the emphasis in sports (and in sport psychology) on the individual, it is important to remember that, in team sports, it is the team that is most important, and that it is the team, not individuals, that wins and loses.

Mental toughness
Helping athletes bounce back from losing, disappointment, frustration, or injury; and overcome other obstacles like anxiety, tensions, and self-doubt.

Coaching methods
Changing the way coaches instruct, motivate, and manage their players.

Does sport psychology have an impact off the field?

The positive thinking, self-confidence, self-discipline, and team/player attitude that comes from playing sports have benefits in other areas of life. People who are self-confident and know how to get the most out of their own abilities and how to help others ("teammates") get the most out of theirs will be successful in anything they do. People are starting to recognize this, and the skills and methods that sports psychology applies to athletics are starting to be applied to other areas of life as well. The message is becoming clear: play sports, have fun, be positive, be a team player, and you will get the most out of yourself in lots of different areas.

What are the mental benefits young people get by playing sports?

There is a lot of research that shows that playing sports, especially team sports, helps both boys and girls build better friendships and be more self-confident and assertive.  These psychological benefits are of course in addition to the physical benefits that come from being active and athletic.  You don't have to be the star of the team, or compete at a high level, to benefit from involvement in athletic activities.

There is sometimes another, darker side to youth sports, though.  In the U.S., organized, overly competitive sports leagues have often replaced pick-up games and just-for-fun leagues even for little kids.  Having fun can take a backseat to winning at all costs, often because parents and coaches drive the players too hard, too soon.  And there are also indications that young athletes shouldn't specialize in one sport before they are in high school.  When youth athletes become overly competitive and overly specialized, they often stop having fun and can lose interest.  Sports can seem like work instead of play.  This is called "burnout," and it is becoming a real problem for some young athletes in some circumstances.  But as long as kids, parents, and coaches have the right attitude towards youth sports, the players can benefit in a number of ways, and these benefits can last a lifetime.