Appreciation is a way of recognizing and acknowledging good work done by somebody. Appreciation letterscan help boost the morale of the concerned parties as well as motivate them to continue to work hard. These letters are generally written in professional and business scenarios.
It is more effective to address appreciation letters to one individual. Even if it is a team you want to laud, you can address it to the Team Manager and request that the appreciation be passed on to other members of the team.
While writing an appreciation letter, make sure to quote a specific achievement or contribution made by the individual/team. If it is for overall performance, mention that. The letters can also include praise and appreciation from other quarters for the concerned party.
It is important to convey your appreciation in a sincere and genuine manner. Keep the tone warm and personal without sounding patronizing. It is best to send out these letters with a hand-written line or two at the end or beginning to make it more personal.