Professional astronomy today consists of large ground-based observatories and satellites, with sophisticated detectors using the most recent technology, and huge amounts of computing power to cope with large quantites of digitized data and image processing. Amateur astronomy relies on more compact, more traditional, more portable, and less expensive telescopes usually designed more for viewing pleasure rather than solely for the purpose of gathering data. That's why so many people choose amateur astronomy as a hobby-- it's somewhat less complicated, which makes it more fun, than professional astronomy.
Amateur astronomy and astrophotography contribute greatly to astronomy in terms of asteroid and comet tracking, and photometry. Furthermore, amateur astronomy is very accessible to the public domain in terms of costs and educational outreach. Even if you don't own a telescope and aren't an astronomer, basic stargazing is easy to get started in, since the only detector you need are your own eyes.
Note to our hometown crowd in L.A. and to folks in other light-polluted cities: The sky may be too bright for amateur astronomy in the city limits, but even in L.A. you can discern the brightest stars and constellations, and still observe almanac events (moon/sun rise + set). Darker skies are only a 1-2 hours drive away at sites such as Mt. Pinos, Red Rock, and Joshua Tree.
Amateur Astronomy Organizations