Belief is one of the most important concepts in Planescape. The Outer Planes are the personification of belief, if one believes strongly enough then reality on the Planes adjusts to match. This is the key behind the power of the factions, and the reason why the Outer Planes exist as they are and why fiends and celestials exist as they are. Belief moves entire layers of planes from one plane to another.
Belief is not as simple as closing your eyes and pretending strongly that something is as you wish it to be. (Though a Signer at first glance may tell you otherwise.) Belief is about a solid philosophical foundation, a personal understanding of the Multiverse and of generally 'how things work'. With the conviction that things are as they are, everything else falls into place naturally.
Your version of how things work may not match the next persons, but given enough belief in it you both may find yourselves affecting the multiverse around you. This core element of reality in the Planes is the reason behind the existance of the Factions. Of course, the need to make others understand how things really work is also the reason why the Factions originally centered their activities around Sigil. Rumor holds it that if one particular belief came to dominate that central location in the planes, then all the rest would follow.
Sources
Planewalkers Handbook (2nd ed.)
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