Frequency is the number of repetitions of a periodic phenomenon happening in 1 second. It is measured in Hz (from physicist Hertz).
For example 440Hz means 440 repetitions in one second.
90.2Mz on FM radio means electrons move up and down inside the transmitter and therefore the receiver antenna 90.2 million times per second.
Pitch is how high a sound is perceived by our brain. It is measured in Mels (from the word melody).
Pitch is obviously related to frequency but they are not the same thing. Frequency is objective, pitch is subjective.
For example pitch does not only depend on frequency but intensity too. For example when playing a 440Hz sinewave at higher intensity our brain perceives a lower pitch.
Other example is how 880Hz although it is a mathematical or engineering octave sounds flat to our brain which usually requires just a bit more in frequency. How much more depends on many variables. What frequency, at what intensity etc.
See also:
Sound and Hearing - Life science series - Stevens, Warshofsky
Musical Acoustics - Donald Hall
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