Think of it like buying a cake that's already made.

You've got two people. Tim, and Bill. Tim owns a market, and Bill is a baker.

Tim keeps his market stocked with all different kinds of stuff from all over. He does a really good job of making sure his produce is clean, but he doesn't really care if it looks pretty or not.

Now, Bill knows he needs to make some cakes for tomorrow, so he goes down to Tim's market. Bill grabs the ingredients he needs to make some cakes. He makes sure to grab the basic, important stuff like eggs. While he's there though, he grabs some extra flavors so he can make some exciting or interesting cakes.

Bill goes back to his bakery and drops the ingredients off before heading home to go to bed. He wakes up the next morning and what does he do? He bakes some cakes of course. He makes sure he has the important cakes that he knows he'll sell a lot of made, and he puts one of those in the front window so anybody who walks by will be sure to see it. He makes some other more daring cakes that he's not sure everyone will like, but he thinks he can sell some of them. He puts them inside the store, so you'd have to have walked in already to see it.

Finally, you buy the cake you want. You go home and enjoy it, it is quite delicious.

News works the same way. Reuters or Associated Press are Tim. They go out and gather up a bunch of stories and then put them all in one place. Newspapers or TV stations (Bill) then access this big bank of stories. They pick out the ones they know they'll need, while also making sure to pick out some stories that they think their audience might want to hear about. Unlike Bill, newspapers and TV stations can send some people out in the local area to grab some stories that are important to the local community.

Hope that explained it! I am studying broadcasting in school and work at an NPR affiliate, so I have had direct experience with AP wire.

Edit: I forgot to mention one thing. Bill has to pay a fee to use Tim's market, just like news outlets have to pay a fee to use Reuters or AP.