Letter Of Recommendation (LOR) is a mandatory document for your MS or PhD applications. You can get it from your academic Professors (preferably with a PhD degree) or Industry experts, under whom you should have studied/worked. Universities prefer to see LORs from academic Professors rather than Industry experts.
Some Universities need only 2 LORs, while some need 4 LORs. You will have to find 5 people to give you a LOR. The reason for 5 people is because, if you are applying to more than 10 or 12 Universities - you could reduce the work for your recommenders. Generally LOR filling/uploading is a time taking task for busy Professors / Industry experts. Try to help them help you. Plan it well in advance.
Some recommenders prefer to type a LOR for you on their own, some may ask you to type a LOR and bring it to them. Whatever be the case - try to help them to the extent possible with whatever help they need. Because they are carving your future.
Always ask LORs from people whom you trust. Never ask someone whom you don't know that well, they will end up writing a mediocre / poor LOR which can affect your chance of admission.