Is it about the meaning of the sentence or the type of name that comes after it?
"Ça" can replace "Cela" in all sentences, it means the same thing. "Ça" is more used in familiar language and "Cela" in formal or literary language.
ça is the contraction of cela just like c'est is the contraction of cela est. However ceci and cela are differents like this and that.