Water is polar meaning that it has prominent areas of negative and positive charge. Among water molecules there is a weak force called hydrogen bonding which causes the molecules to "stick" to one another. This sticking to itself is called cohesion, and it causes water to ball up into droplets. This same force also causes water to stick to other kinds of molecules. This is called adhesion, and it is what causes water to ride up on the side of a water container slightly.
