SQL & PL/SQL

Hello,

Can someone please explain to me what is the difference between:

SELECT REGEXP_INSTR('aoeeeeo','(i|e)o') position FROM dual;

and

SELECT REGEXP_INSTR('aoeeeeo','[ie]o') position FROM dual;

I mean, what is the main difference between round brackets with the alternation operator and square brackets in regular expressions?

Thank you very much,