I am currently an engineering undergrad student with a career fair approaching(early February). One of my professors offered to write a letter of reference if i ever needed it after seeing my performance in his class. I am not familiar with a letter of ref though, are they commonly specific to a company? or simply something that is universal(essentially the same as your list of references). Also, should i include this professor on my list of references even though i also have the written letter? Any advice is appreciated, as you can see i am clearly lacking knowledge in this field.