Comment by osoba

U, V: vector spaces

u, v: elements of vector spaces

G, H: groups

g, h: elements of groups or

g, h: homomorphisms, isomorphisms etc

e: group identity

K, F: fields

I: ideals

f: functions

(x): sequences

x_i: i-th element of a sequence

A, B: matrices

It all depends on the context it's used of course