Let's take a situation:
I want to buy 100 Microsoft shares. Let's say current going rate is 40 dollars per share. I find only one guy ready to sell at 40 dollars but has only 50 shares . I know there are others who have it but wont sell at 40. Why? They want to sell at higher prices or they are just plain lazy. Whatever but I am desperate to buy. Why ? Because my analyst says there is a chance that Microsoft may acquire Google http://cdn.traderslaboratory.com/forums/images/FH_Sahm/smilies/custom2/laugh.gif. So I raise the offer price to 41 dollars. Immediately I find 2 more guys offering 10 shares each. OK so I bought 70 shares for 2820 dollars @ average 40.28 dollars per share. But I am still short of 30 shares ! Now I raise the offer price to 42 dollars. My order gets filled. So finally I spent 4080 dollars to buy 100 Microsoft shares. That is on an average 40.8 dollars a share.
This whole chain of events happens millions of times on an exchange in seconds and that is how we see prices jump up and down. When some analysts give a buy signal on a stock there will be some analysts who will contradict. Whichever group wins the vote tilts the price that way.
I hope that really satisfies the query.