There are many different airlines in many different countries; some which fly internationally and there are those that do not. As far as here in the United States, there are an abundance of US airlines that fly all around the various parts of the world as well as most all points domestically.
For us here in the United States the major US airlines are all well known. Names like American, Delta, Northwest, TWA, Continental, etc are well know both here in the U.S. and abroad. And since these US airlines cover the bases of most travel plans both here and abroad you should have no trouble finding a US airline that can accomodate just about any travel plans whether they be locally (domestic) or foreign.
As you would expect with US airlines the domestic flights will be much cheaper that those that are intercontinental. So, it only stands to reason that a flight to Venice Italy is going to be more pricey than a flight to Venice, California. Most all of the US airlines will have similar pricing structures... but not the same so you'll want to peruse each of those that seem to fit your travel plans carefully.
If you become confused by all the options or would just rather have a travel agent book your flights for you, you will find that many of the US airlines will be able to accomodate your travel plans. Some of the US airlines will even have partnership deals and arrangements in conjunction with hotels, car rentals, and even other airlines that can sometimes even sweeten the deal for you.
These types of deals can really come into play if you are one that accumulates frequent flyer miles. By taking advantage of these partnerships that some of the US airlines have with various other companies you can come up with some great deals or at the very least enhance the travel deal you've already come across.
This also brings up the point that you should be mindful of your frequent flyer miles when you are looking at booking your flights. You always want to be on the look out and inquire into what you can use your miles for and what travel benefits they'll provide.
No matter if you are traveling across the country, or from one country to another the available options within the various US airlines and the packages they offer, you should be able to put together travel plans that both fit your needs and are affordable.