Technically, preamps and power amps are very similar. They're both circuits within an amplifier designed to elevate the voltage of a signal so that it can be processed appropriately by the proceeding amplifier stage or speaker. The preamp amplifies your guitar's signal to be received and further amplified by the power amp. When the signal has been appropriately processed and amplified by the power amp, it's sent to drive the speaker(s).
Generally, preamps feature some type of tone stack and gain control or preamp volume, or both depending on the amplifier. Power amps are typically more basic; some feature only a volume control, while others boast a simple tone stack. In this way, preamps and power amps have a lot in common, but each circuit has its own unique tonal contribution to sound of the amplifier.
Most amplifiers utilize the preamp circuitry for the bulk of the amp's tone. The majority of the tone-shaping takes place here because in most cases, the preamplifier circuit contains more comprehensive tone-shaping options. In general, preamps feature some type of equalizer and drive control. Some amplifiers like the Mesa Boogie Dual Rectifier, for example, rely much more exclusively on the preamp to generate distortion and tone, but almost all amplifiers rely on the preamp to provide the bulk of the amp's breakup. In general, preamp distortion is more compressed and smoother sounding. It has a warmer, fizzier tonal quality, and it sounds similar to an overdrive-style pedal, and different types of preamp tubes can change the tonality completely.
Amplifiers like the Marshall JMP series, however, rely on the power amp to add an additional layer of mushy, gushy, harmonic, goodness. It was this sound that characterized many of the guitar tones heard during the '60s and '70s, including those of Alex Lifeson's, which happen to be my personal favorite. While most power amps feature nothing more than a volume and sometimes a small tone-stack (usually just a "presence" or "brilliance" control), they can add a significant amount of charm to your sound. Power amps break up much later than preamps do, and have a significantly greater output. This means that power amp volume has to be elevated to about 50-75% (give or take, dependent on many factors) before the power section begins to break up. For amplifiers with higher wattages, this means that power tube breakup can only be had at ear-splitting volumes.
Drive coming from the power amp is gnarly, greasy, and has a harder break-up. Different types of power tubes can also have a huge tonal impact. A 6L6 power tube, for example, has a deeper, more pronounced low section and harsher, less even break-up. EL34 power tubes, on the other hand, have more pronounced mids and highs, and a smoother distortion that generally yields more gain.
I hope that this adds some clarity in regard to the similarities and differences between an amplifier's preamp and power amp. Happy strumming!