One definition I came across is "a belief is an assumed truth". We create beliefs to anchor our understanding of the world around us, to "make order out of chaos". Once a belief is formed we tend to keep on believing it, even in the face of dis-confirming evidence, particularly if others know of the belief, but mainly because it's been woven into a web of wider beliefs and disbeliefs. We usually believe that things will happen as they have previously happened, this means that everything is a belief. Our beliefs are highly connected to our language, if there is a word for it then we can believe it. Thus we "language reality into existence".
When we believe in one thing, we usually disbelieve in anything that contradicts the fundamental belief. This results are one fundamental belief system, and many disbeliefs surrounding it. For example a person believing in one religion, disbelieves in all other religions, some call it "contempt prior to investigation" but it's really disbelief.
If a belief is fundamentally an assumption then it can be changed. It's important that to change a fundamental belief we also must change the disbeliefs surrounding it. In order to do this we must understand where the belief came from, and how it was formed. Beliefs are formed in two ways. Our own experience and reflection or as blind acceptance to what others have told us. Usually when we were young and had no knowledge to dispute it. Relying on faith that the other knew.
When a persons belief appears based on experience, try exposing the experience as limited, "so far that's been your experience, does that mean it will always be so? What about others who believe differently, and have seen their beliefs to be true?". On the other hand when the belief is based on blind acceptance, you challenge the credibility of the source, "where did this belief come from...your Mother? So where did her knowledge come from, what qualifies her to be able to say that?"
Credibility is very important here before you start doing any persuading. Credibility should be used to gain commitment, never argue a case. Credibility is context dependent and an expert in one situation may be incompetent in another. You can enhance credibility by highlighting your own experience, showing that you care and that your the same as them, and by leveraging the credibility of your sources of information. Credibility is reduced by making excessive exaggerations, qualifying or making disclaimers. To change beliefs it's important that your advisors have credibility and nothing to gain from your decision.