String theory  is a part of physics that has wiggled its way into the popular consciousness recently. While the details of string theory are very complicated and full of math, the basic idea is stunningly simple. Namely, that the fundamental Lego blocks which make up all matter and all forces are tiny vibrating strings. These strings are incredibly small, too small to be seen even in a microscope.
This idea that strings are the fundamental Legos is hard to grasp, at first. After all, we're used to thinking of an average piece of guitar/violin/piano or cat string as being made up of atoms, which themselves are made of subatomic particles. These tiny, vibrating, fundamental strings are different than ordinary string, though. Fundamental strings are the stuff out of which everything is made. The idea is that even a single electron in a carbon atom in your body is composed of fundamental string, and so is a photon of ultraviolet light!
A familiar violin string can give rise to a variety of musical notes, by vibrating in different ways. Similarly, a tiny fundamental string can vibrate in many different ways, corresponding to different subatomic particles. Fundamental strings can also be open (with ends), or closed (joined up, no ends), which makes them more 'musically' versatile. Poetically, then, we might call string theory 'Nature's symphony'.
String theory is a rather economical idea, at many levels. But that is not the only reason why string theorists like it. Another part of the story involves two great pillars of 20th century physics: General Relativity (GR) and Quantum Mechanics (QM). GR is the physics of very heavy things, while QM is the physics of very small things. Both GR and QM are exquisitely well-tested by experiments to date. The trouble is that GR and QM cannot be combined consistently at a theoretical level, for things that are both very small and very heavy, like black holes and big bangs. That is, If you assume particles are the fundamental LEGO blocks of Nature. If you pick your LEGOs to be fundamental strings instead, that does the trick beautifully, for making a consistent theory of quantum gravity applicable at all distance/energy scales.
Gravity is my favourite force (skiing would, like, suck without gravity!). Figuring out the quantum behaviour of gravity is my research interest. String theory provides the best theory of quantum gravity available, so my research investigations naturally take place within this context. To test ideas about quantum gravity, I do 'thought experiments' on what I like to call extreme physics - the deep interior of black holes and the birth of the universe.
As yet, strings haven't been seen experimentally, but physicists and astrophysicists are hard at work figuring out how stringy physics could be detected, either directly or indirectly. There are two avenues of interest: experimental particle physics and experimental cosmology. Experimental particle physics involves cranking up, while experimental cosmology involves sifting through the fine details of the left over from the "big bang", the violent cataclysm that formed the Universe.
 The usage of the word theory in physics is different, and much more specific, than the ordinary usage. Also, a scientific theory is produced via the scientific method, and is not just some arbitrary notion.
Each time I prepare a talk electronically, I provide the audience with an electronic copy of my slides at a specific (unadvertised) online address. Those files are still extant.
Stuff related to my research interests:-
What is Spacetime?by George Musser
Black Holes and the Information Paradox, by Prof. Leonard Susskind
ER=EPR, or What's Behind the Horizons of Black Holes?
Other good physics resources online for laypeople:-