If you are planning on applying to graduate school, you should also be planning on requesting letters of recommendation. Most graduate programs require at least two letters of recommendation. Important pieces of your application include transcripts, standardized test scores, and a personal statement/essay, in addition to a glowing letter of recommendation. The letter of recommendation can give admission committees a picture of you that the other pieces of your application cannot.
Definition of a recommendation letter--A detailed discussion, from a faculty member, of the personal qualities, accomplishments, and experiences that make you unique and suited for the program to which you have applied.
Typically, one or more of the people in your network and/or your mentors will write your recommendation. It is important to be consistent in communication with them about goals and needs. The more familiar they are with these the better the recommendation they can write. It is important to get your request in for a recommendation early, as it takes a significant amount of time and, often, yours is not the only recommendation they are writing, and they are all usually needed around the same time.