Theoretical physicists are rather typical scientists. If you imagine them as absent-minded, egg-headed, bizarre characters scratching their chins while deeply engaged in thought... Well, most of the time you'd be right.
What these people do is to try to figure out how Nature works. That is, why the stars shine, why water is fluid and the sky is blue, what you are made of and why does "it" weigh that much, why the universe expands, or what energy and matter actually are...
One of the most amazing facts of human life is that scientists can actually answer some of these questions, with the answers progressively becoming better and better (that is, being more and more in agreement with observation). Perhaps the fact that we humans enjoy poetry and music is, come to think of it, the only other fact which is equally surprising.
Two questions are often asked: "what is the difference between a theoretical and an experimental physicist and why is there a difference?''. The answer to the second question is simple: the two "species'' do quite different and often very specialized things. It is increasingly hard to find people like Leonardo da Vinci, who know and are active on "everything''. The experimentalist interrogates Nature directly, by observing it passively, like astronomers do, or actively, like particle experimentalists do, in "playing'' with Nature's smallest constituents to figure out directly how they behave.
The relation between experimentalists and theorists is often one of healthy competition for truth and less healthy competition for fame. Here is a riddle reflecting that fact:
What is similar and what is different between the following two sets?:
The first set consists of a farmer, his pig and the truffles:
The second set consists of the theorist, the experimentalist and the big discoveries:
(you can see by my drawing of the second set that I am not an experimentalist).
The answer to the riddle is:
The farmer takes his pig to the woods. The pig snifs around looking for a truffle. When the pig gets it and is about to eat it, the farmer kicks the pig on the head with his club and steals the truffle. Those are the similarities: a theorist would also claim recognition for an experimenter's discovery (if it has anything to do with her/his theories) even if [s]he did not make it!
The difference is that the farmer always takes the pig to woods where there are truffles, while more often than not, the suggestions by the theorists take the experimentalists to "woods'' where there are no "truffles'' (by suggesting experiments that do not lead to interesting discoveries).
Not to be unfair to theorists, one must add that there are notable exceptions to these rules, progress is made by trial and error, and the theorists' guidance is occasionally in the right direction! Even more often, while looking for the theorists' "truffles'' the experimentalists find "gold'': something unexpected but even more interesting! (Nature tends to be more creative than we are).
The main specialty of theoretical physicists at CERN is trying to understand "elementary particles", which are the fundamental constituents of the Universe and the agents of the basic forces of Nature, like gravity. As it turns out, our ever-advancing knowledge of these "elementary" little things is also the basis of our understanding of the Universe as a whole!
If you made a short visit to CERN’s Theory Division you might think that you are in a zoo. But that is not *entirely* right. True enough, you will find women and men of dozens of nationalities, cultures, languages and what not... some of them may even look like ET. But what these people are doing is what defines our species in its ensemble: asking, and sometimes answering, some of the deepest questions. Thus the zoo is more like a circus of magicians, in which the performers - uncharacteristically - would insist in showing you their cards... and the entrance is free!