The iPad includes an internal microphone and speaker; there is also a socket to connect an external microphone or earphones, there is no USB port. The two ways to connect external audio devices are:
Use the headphone socket
The headphone socket on the iPad allows the connection of external headphones. The socket actually includes the facility to connect an external microphone but this is wired in a non standard way and so plugging in a normal microphone will not work. Various adapters are available to allow use of external microphones; these range from a simple adapter for a standard cheap microphone through to a device that will enable the connection of a professional balanced line XLR microphone or mixer.
Use the Apple Camera Connection Kit
However, the adapter will also allow connection of other audio devices such as USB microphones, MIDI keyboards etc but this is not officially supported. Using the adapter in this way has been found to work effectively with some devices but not with others. It appears to be trial & error as to whether a particular device will work.
A summary document that shows various audio connection options is available here: