While in Mathematica, you may pull down the "Help" menu and select "Open Function Browser...". A window will appear with three check-buttons at the top, with the one labeled "Built-in Functions" initially selected. Below that are three scrollable subwindows for topics headings in the browser. Click on "Numerical Computation" in the left subwindow, for example, and the second subwindow will fill with subheadings appropriate to that topic. Click on "Equation Solving" there, and the third window will fill with names of relevant functions. Click on "Solve" there, and the subwindow below will display the brief documentation for that function. (The same documentation can be obtained directly by entering "?Solve" into a cell and evaluating the cell, but the browser helps you find the name of the function.)

Another way to use the function browse is this. Suppose you have typed the word "Solve" in a cell, and want to see the documentation on it. Use the mouse to select (highlight) the word, and then pull down the "Help" menu and select "Find in Function Browser". The brief documentation will appear.

The online help in Mathematica version 3.0 is much better: the entire Mathematica book is there!