The lines commonly seen on weather maps are called isobars. They are used to show areas that have the same barometric pressure, either at a given time or on average over a certain period.
Aside from isobars, there are other lines used on weather maps to illustrate geographical relationships. An isothermal line, or isotherm, for example, functions much like an isobar. It shows which areas on a map have the same temperature. Lines also can denote cold, warm and occluded fronts. Cold fronts usually are marked by blue lines, warm fronts by red lines and occluded fronts by purple lines.