Or, in other words if you are defining something, you will most likely use "c'est".They are used almost interchangeably in spoken conversation, as far as I understand. The functional difference is related to the notion of topic versus comment. If a person or object has already been mentioned in a conversation, you'll probably use il est/ elle est. (Elle est musicienne) If the person or object is being introduced in the conversation for the first time, you are more likely to use c'est. (Julie, c'est une musicienne)
2) Otherwise :
2 a) If what comes after "est" is an noun attribute, always use "c'est".
(Voici Paul, c'est mon ami : (Mon ami) is a nominal group)
This rule is very powerful. The cases given by Tesley (profession, etc.) are only a very small subset of the possibilities.
2 b) If what comes after "est" is an adjective attribute, always use "il" or "elle".
(voici ma bicyclette, elle est rouge)
2 c) If what comes after "est" is not an attribute, always use "il" or "elle".
(Où est le livre? Il est dans la cuisine).
In this link, there are a few mistakes...
The worse being: "Ce sont très loin." which is totally incorrect !
You might say: "C'est très loin" (if speaking of a defined place)
or " Il / Elle est très loin." _ " Ils / Elles sont très loin." depending on the context.
Also for this example given on the link: "C'est bizarre, ce livre."
I would never say that! I would say "Il est bizarre ce livre." Here "il" means "that book" ("livre" is of male gender in french) as in "That book is weird". You add "ce livre" at the end of the sentence to define what "il" was referring to.
But "C'est" would mean "It is weird, that book." I don't know how this sentence sounds to English natives, but it sure sounds weird to me! And so does it in french