Oil energy is used in a variety of contexts. The most familiar to people probably is the gasoline that you put in your car, which is derived from oil. Trucks, trains, ships and aeroplanes also run on energy from burning fuel, and there are a number of oil power plants as well. While it is extremely common, this doesn't mean oil energy is perfect, and there are many pros and cons involved in its use.
One major positive aspect of oil energy is that oil is relatively common. This doesn't mean it's infinite, but there are a number of places where oil can be drilled from, such as offshore rigs in the ocean, the tar sands in Canada and numerous underground deposits in the Middle East. Energy needs to be accessible to be effective, and this is a major positive attribute of oil energy.
Oil is a liquid. This means that it can be transported more easily than solids or gases, as it simply needs to be poured into a container. This is the reason it is possible to run your car in Iowa on oil drilled in Iraq -- the oil can be drilled, then loaded into a supertanker, then unloaded at any port in the world. Again, this has to do with its accessibility. Oil's dispersal across the globe makes it accessible for drillers, and the liquid form it takes makes it accessible for consumers.
There is a finite amount of oil on the planet. This means that it can be tapped out, or at the very least made less accessible as the easy-to-access sources dry up, leaving drillers stuck with harder, more expensive-to-access sources. This is a major con of oil energy -- it's not going to be around forever.
While oil is easy to transport in supertankers, this also raises the environmental stakes -- if a supertanker runs into something it will leak oil all over the ocean, killing wildlife and changing the ecosystem for decades. What's more, the process of burning oil generates a lot of pollution by releasing sulphur and carbon dioxide into the atmosphere. So even if transporting oil goes off without a hitch, the actual act of using it pollutes the world anyway.