Goal Seek is fairly straight forward, (is the only one i know of these! haha)
lets say you have a whole bunch of data and equations. Lets say if you change the value in cell A1 the formula that carry through from this cell or the range that includes it, will also change as they recalculate based on your new value.
Now lets say you have a bunch of numeric values throughout column A. In column B you have a formula to give the participation of each figure in relation to the whole data set, eg.
...cell B2...
=A2/SUM(A:A)
so that against each figure you can see it as a percentage of the group.
Now imagine you want to change the cell A2 value so that the participation becomes exactly 35%. You go to goal seek function, and tell it to change the contents of cell A2 until the value in cell B2 is equal to 35% (or 0.35), and it will calculate the required value for you.
hope this gives you a head start, if you create an application for it where formula operate on other formula, it can be quite powerful.