Well the two aircraft are very different in both appearance and mission orientation. I will give you the differences below and some links to help you out.
The McDonnell Douglas F-18 Hornet: This plane was designed in the late 1970's for the United States Navy as a Multi Role fighter to replace the Grumman F-14 Tomcat. The F-18 is light weight and agile dual engines dual fins and unswept wings. It is used for fleet defense and can be used for offense also. It is Carrier and land capable giving a greater use in various roles. Can be crewed by one or two crew men. To become an F-18 Hornet pilot you must go to the Naval academy and then apply for flight status on the F-18. Then during training you must accomplish a certain number of Carrier landings before you can be given full flight status. If you fail you are washed out of the flight program and have to wait at least a year or more to try again. The F-18 Hornet is used only be the United States Navy and United States Marines. The F-18 is also exported to some countries like Australia, Switzerland, Kuwait and 4 other countries. There are some other countries that may buy but the sales haven't come trough.
The General Dynamics F-111 Aardvark: This plane is designed as an Interdiction Bomber/Elint plane. It is larger than the F-18 Hornet and far faster than the F-18, and has a greater weapons load. It also has 2 engines but one stabilizer rather than 2 on the F-18 Hornet. The F-111 has what is called Swing Wing or Variable Sweep, meaning that the wings can be moved fulling forward for slow fights and take off and landing. Or Swept back for Super Sonic flight. This allows greater weapons loads and substantially better range. This plane is piloted by a crew of 2 seated side by side, and one of the crew has what looks like a TV set which is used to guide Air to Ground Missiles to the intended target. The Elint version of the F-111 uses special electronics to jam enemy radar and other communication devices to make it harder for the enemy to fight and see incoming aircraft. This plane is only by the United States Air Force and has now been retired from service. It is not Carrier capable though there was an attempt made in the late 1960's, trough the mid 1970's to make one but never was purchased by the Navy. It was made for export to Australia. but has now also retired their fleet in anticipation of the F-35 to take its place. Below are some links to sites that will help you with your quest for information on both aircraft.
http://www.militaryfactory.com/aircraft_comparisonx4.asp
http://www.vectorsite.net/avf111.html
http://www.aviastar.org/air/usa/general_f-111.php
http://www.vectorsite.net/avhorn.html
http://www.aviastar.org/air/usa/mcdonnel_hornet.php
http://en.wikipedia.org/wiki/General_Dynamics_F-111_Aardvark
http://en.wikipedia.org/wiki/McDonnell_Douglas_F/A-18_Hornet