I know this is a stupid question but I've heard people talking about Ubers. What are Ubers, how do you know something is an Uber, and what makes something an Uber?????
Ubers refer to Pokemon within the Uber tier. They are the best Pokemon there are, and mostly consist of the legendary Pokemon with really high stats.
Here's a list of the Ubers, for a basic idea.
Users are usually Pokemon with really high Base stat. Usually offensive, defensive, or supporters. The majority are legendary, excluding wobbufet. Offensive Pokemon like Rayquaza have high atk or s.atk can and can set up and sweep. Pokemon like Lugia have high defensive and hp stats. They can wall Pokemon and stall them out with hp recovering moves. Supporter Pokemon like mew can set up and help other Pokemon sweep.
Here's a link for all the Ubers for 4th gen