Let's look at Northrup Grumman's Antares Launch today.
At the time of this blog post being written, Northrup Grumman's rocket named Antares has not launched yet and is currently being fueled up on the launchpad ready for launch, meaning we do not know if the Antares/Cygnus launch has been aborted and/or scrubbed yet. Cygnus is the pressurized module atop the Antares Rocket second stage. A pressurized module is a spacecraft module that humans can live in, as there is breathable air. Unpressurized spacecraft modules are modules that do not have breathable air in it and are a vacuum. The word vacuum means empty space, meaning an unpressurized module contains no matter.
The Antares rocket uses Liquid Oxygen and Kerosene for fuel on its first stage/booster stage. The first/booster stage uses two twin RD-181 Engines.
The second stage uses a solid rocket motor called Nothrup Grumman CASTOR®.
Liquid Fuel is used on the first/booster stage because it is easier to control than solid fuel, and has less limitations. You are able to throttle and stop liquid fuel engines, but you cannot throttle or stop solid fuel engines. Solid Fuel is used on the second stage because the main work to get into space has been done by the booster stage and does not require all of the liquid fuel engine features. Solid Fuel is also very cheap, which is why the Space Shuttle used Solid Fuel, and why the future NASA Rocket, SLS, will also use Solid Fuel. Lots of other spacecraft vehicles like Vega, the Ariane 5, the Atlas V, United Launch Alliances' future Vulcan-Centaur Rocket, and much more, also use solid fuel.
So far, everything is go for launch.
UPDATE: The launch has been aborted due to ground equipment issues, and not an issue with Antares itself.
ALSO ANOTHER UPDATE: Nearly 24 hours have passed and Antares has successfully lifted off and is now on its way to the International Space Station where it will arrive in about 3 days on Monday. The ISS Commander Chris Cassidy will use the ISS Canada Arm to dock it with the ISS and retrieve the cargo.