Imagine yourself in an interview with the admissions officer and you have to convey that you are the perfect applicant with the relative qualification and will be a perfect student who is hard working and dedicated.
Letter of Recommendation needs to convince the admission officer about your qualification and dedication. It should provide the admission committee enough information to create a positive impact.
They should be on the referee’s/organizations letterhead with the respective stamp/signature.
Choose your referee wisely; they should have known you in academic and/or professional capacity.