When is a noun regular vs irregular?

Question

Hello, When is a noun regular and when is it irregular?Nouns that end with an -o or -a are regular and everything else is irregular?

Answer

Normally nouns ending in O are masculine and those ending in A are feminine.

However there are nouns ending in O that are feminine such as foto, mano and radio. There are also nouns ending in A that are masculine such as clima, idioma and día.

There are lots of lessons on noun endings you can refer to on here.

Answer

Hola David
We call a noun a regular noun when the ending is -o for its masculine and -a for its feminine form. But as Marsha said, there are lots of exceptions and different endings that don't follow the rule. 
Here are two links where you'll find lots of lessons about gender (click and scroll down to find the lessons)
Feminine gender
Masculine gender
I hope it helps
Inma