An acceptance or admission letter, sent to a student, is the proof that the student has been offered a place for the selected program of study at the institution where the student had applied for admission.
If you need more explanation then I would like to say that if a student is interested in studying abroad in the UK then he/she has to start the process with an application to a college or university in the UK. Anyone interested should go on the internet and check the Webpage of a college or university for admission to a certain course/program of study. On the webpage of a university or college a student will find all sort of information for the prospective students. One can check if the desired course or program is available at the institution or not. If the program that interests a student is available then he/she should go further to know about the admission requirements for this specific program and the application deadline. The interested student has to download the application form or apply online for this program and provide all the required documents to the concerned department. After this the duty of the applicant is done.
After all this the student should wait for a response from the college or university. In the next few weeks the concerned department would send a letter of admission if the student has been admitted to the program and if unfortunately the admission has been refused, for some reason, then this letter is called refusal letter and this letter of refusal means nothing for a student.
Now I think you have got the point. The admission letter or the letter of admission or the acceptance letter or the letter of acceptance, now also called visa letter, is a sort of invitation from the educational institute and with this letter you should apply for your visa. Remember that this letter is not the sure guarantee of your visa. For your visa you have to apply to the British High Commission, along with your visa/admission letter.
The person in the college or university who has sent you this letter is called your sponsor for sending you the letter.
After receiving your admission letter, you should apply for visa and this is the most important step. So be careful and provide all information correctly or your visa will be rejected.
If you cannot go through all this process then you should contact an educational consultant in your city for your admission at an educational institute in the UK and for help to arrange your documents before you apply for visa. But I am sorry to say that most of the education consultants are frauds. So make it sure that your consultant is not fraudulent.
For this purpose you should contact the British council near you as it is their duty to help students who want to study in the UK.
The British council webpage http://www.britishcouncil.org/