Letters of recommendation are part of your graduate school application and are required by all graduate programs. Your letters of recommendation play a significant role in the admissions process. Therefore, it is important that your letters be from professors/mentors that know you well.
Below are resourses to help you approach potential letter writers.
Asking for a Letter of Recommendation
PDF file from UCLA with tips on asking for letters of recommendation
You should include this with your CV, Statement of Purpose and any other material that supports your graduate school applications. Providing this information will help your recommenders write a strong, personalized letter of recommendation that will make you a more competitive applicant.