Dikes and sills are both types of igneous intrusions, but they differ in their orientation and formation.
Dikes are vertical or steeply inclined walls of igneous rock that cut across existing layers of sedimentary rock. They form when magma intrudes into fractures or cracks in the surrounding rock and solidifies. Because they intrude perpendicular to the existing rock layers, dikes can vary widely in thickness and may be found in a range of environments.
Sills, on the other hand, are horizontal or gently inclined sheets of igneous rock that form between existing layers of sedimentary rock. They occur when magma intrudes parallel to the bedding planes of the rock above and below. Sills can be quite extensive laterally and tend to be more uniform in thickness compared to dikes.
In summary, the main difference is their orientation: dikes are vertical intrusions while sills are horizontal. This difference not only affects their formation but also the geological features they create in the surrounding rock.