There are conventions, but no standard set in stone. For instance, mathematicians like to use m, n to stand for integers, r - for reals and rationals and f, g, h - for functions. However, you can use any symbol to denote any object or idea.
For another example, one of my Measure Theory textbooks denotes outer measure of a set A as |A| whereas another one uses m*(A).
Consider the following partial list of symbols that occurs at the beginning of Dummit&Foote's Abstract Algebra[0]. This tells you nothing unless you know the underlying ideas these symbols represent. This list is there to tell you what kind of conventions they would be following because different symbols might be used in other books.