Continuing from Angua’s explanation, one can further say that some scientific work is more observational than theoretical or experimental. Although all experimentation implies observation (or else what’s the point?), there are some circumstances in which one can merely observe and not control the variables. An example would be astronomy in its most common form (I’m not trying to step on Angua the Astronomer’s toes here!).
As a case study, consider the identification of helium in our own sun. 19th century astronomers such as Joseph Norman Lockyer and Pierre Janssen noted (via observation) that the sun’s emission spectrum contained atomic lines that could not be accounted for by any known terrestrial element. They postulated that the sun contained a new element, which they called “helium” after the Greek word for “sun”. Decades later, laboratory experiments were carried out on the gas that we now know as helium, and the atomic emission lines were found to match those in the previously-observed solar spectrum. Bingo! The combination of observation and experiment identified the gas in the sun as the same one as found (in much smaller quantities) on earth. Later theoretical advances (such as quantum theory) enabled scientists to identify exactly which transitions in the helium atom were responsible for the various observed spectral lines.
Think of observation, theory, and experiment as the tripod that holds up scientific research. Without any one of them, the endeavor would fail. As Angua said, without experimental results, the theoretician has no way of verifying his/her computer models. There’s a lot of overlap between disciplines, however, and although one can find people who claim to be “100% theoreticians” it’s unusual to find “100% observers” or “100% experimentalists”. Even “lab rats” (of the human variety, that is) run simulations from time to time.
For a current astronomical mission in which the experimental, observational, and theoretical aspects are easy to identify, take a look at NASA’s “Deep Impact”. This is a probe (launched January 12, 2005) that will make rendezvous with comet Tempel 1 on July 4 of this year (it’s a US mission, and the date is no coincidence…). It’s not just going to observe the comet, however; the probe will launch an “impactor” that will smash into the comet. As it approaches the comet, the impactor’s instruments will relay images back to the “flyby spacecraft”. After impact, the resulting ejected matter will be observed by the flyby spacecraft, and the results sent back to Earth, where the data will be analyzed. This will lead theoreticians to postulate new models of what is occuring in the comet (and what caused its formation), and experimentalists will devise new laboratory experiments to test these hypotheses. The experimental results will refine the hypotheses, and the dance will continue until a next-generation probe is sent to a comet at some time in the future.
[Personal note: a colleague of mine is part of the Deep Impact scientific team, and will be in Hawai’i on July 4 to observe the mission’s climax. I will be involved in the laboratory experiments that will be conducted in the months following the “Impact”. It’s fun stuff!]