Dolerite is also known as diabase and micro gabbro. It is a medium grained intrusive rock with the same composition as basalt.
That definition is a little bit of a mouthful, so let's work through it.
Igneous rocks are rocks which were once molten liquid. As the liquid cooled down it started to crystallise and form minerals. There are a fairly limited number of minerals which are most likely to form in large amounts from the melt. They are all silicates. The most usual rock forming minerals are pyroxenes, olivines, amphiboles, feldspars, and quartz. All of the most common igneous rocks contain some proportion of these minerals, although they are normally not all present in the same rock. Rocks are defined by the relative proportions of these minerals.
The rock forming minerals are quite strongly coloured. Since they are present in large amounts within the rock, the rock is the same colour as its dominant minerals. Rocks with large amounts of pyroxene and olivine are very dark coloured, they generally look black. Rocks with large amounts of quartz and orthoclase feldsar are pale coloured; they can be white, creamy or even salmon pink. Dolerite has high proportions of plagioclase feldspar and pyroxene. Dolerite and basalt have the same proportions of plagioclase feldspar and pyroxenes, but they cooled at different rates. Balsalt generally has no visible crystals, while the minerals in dolerite can be distinguished, just with some difficulty.