Frequency is a physical aspect of a sound and can be measured objectively (for example with a microphone and an oscilloscope). Pitch is a percept of a sound and can only be measured subjectively; often you ask people to adjust one sound so that its pitch is the same/double/half the pitch of a reference sound.
Generally, the perceived pitch is proportional to the "frequency." This is not always true. For example, the perceived pitch tends to change slightly as the loudness/intensity of the sound increases. It is also difficult to define frequency for complex sounds. For pure tones, sin(2pift), the frequency is unambitious, for all other sounds it is more complicated since there is not a single frequency.
The pitch of a harmonic complex sum(sin(2pif0t*i)) where integer i goes from n to m is related to f0 and not nf0 or mf0. In other words, the pitch of a harmonic tone complex is related to its fundamental frequency (even when that frequency is absent) and not its highest (mf0) or lowest (nf0) frequency. When f0 is absent, this is often referred to as the "missing fundamental".
Other pitches, for example Huggins pitch, are even more complicated to describe the "frequency".
