Brussels has a very good metro system, with four metro lines in total. On many maps you will see six lines, but two of these are tram lines travelling overground and then you have a tram line number seven covering Brussels in a big ring. The system is easy to understand and it is easy to get an overview quickly. There are several stops connecting the different metro and tram lines, so you can easily get from one line to another. The general tourist will find these lines more than satisfying and will not need to travel with bus or other means of transportation during their stay in Brussels.
Here you will find a metro map for Brussels. If you click the picture a larger version can be seen as you download a .pdf file.