A recommendation letter is a personal and confidential letter written by a recommender to the admissions committee of the program you are applying to. The recommendation letter serves as an opportunity for the admissions committee to get an opinion about the applicant from a person who have been a faculty or at a supervisory position to the applicant.

Let me just clarify one thing here - The recommendation letter is not a letter written to verify credentials. A weak recommendation letter would be a letter that would simply state the applicant’s achievements. For eg: “…she has secured 70% in my subject and had won the third prize in our technical paper presentation….. she was awarded the national merit scholarship in school ….. she has taken part in our college drama and won the second prize in the college singing competition…” Apparently these things are already evident from your transcripts and your resume. You don’t need somebody to vouch for you. The recommendation should talk about your qualities, your traits, a track record of your improvement, and maybe even your weaknesses and how you have worked at overcoming them. Well there is a lot that’s can be written on what makes a good recommendation letter, but I will leave it for some other time.

For more details on recommendation letters, refer this link

As a closing remark: If the university has provisions for online recommendations, then preferably use them.