In amplitude modulation (AM) , the amplitude of the carrier is modulated as per the message signal. The other aspects of the carrier wave such as frequency phase etc. remain constant. On the other hand, in frequency modulation (FM), only frequency of the carrier wave changes while amplitude, phase etc. remain constant.
Signal quality is a lot superior in FM than AM as amplitude based signals are more susceptible to noise than those which use frequency. Moreover, noise signals are difficult to filter out in AM reception whereas FM receivers easily filter out noise using the capture effect and pre-emphasis, de-emphasis effects.