Neko ga imasu. -> There is a cat.

Neko desu. -> It is a cat.

imasu (living things) and arimasu (objects) -> there is/are

desu -> the appropriate form of the verb 'to be' in English, is/are.

The translation sort of helped me choose when I was learning these, I hope it helps you, too! :)

ga imasu/arimasu is used when you want to explain where is positioned certain person (imasu) or thing (arimasu). Desu is used during formal speech and you can't use it after verbs in -masu form!

Awesome, thanks Rui. Every extra bit helps! =3

Np! :)
I'm glad I could help! 

omg ty! that clears up a lot of things now.