Here's a summary of the classes:

{@link android.nfc.NfcAdapter}

This represents the device's NFC adapter, which is your entry-point to performing NFC operations. Once you acquire an instance with {@link android.nfc.NfcAdapter#getDefaultAdapter}, you can create connections to tags.

{@link android.nfc.NdefTag} and {@link android.nfc.Tag}

These objects represent a tag. A tag is a thing that the NFC-enabled device can communicate with when within range. Usually, you'll work with {@link android.nfc.NdefTag}, which represents a tag that's compliant with the NFC Data Exchange Format (NDEF); a {@link android.nfc.Tag} represents a generalized tag. Note that {@link android.nfc.NdefTag} extends {@link android.nfc.Tag}.

{@link android.nfc.NdefTagConnection} and {@link android.nfc.RawTagConnection}

These objects represent a connection to a tag, respective to the type of tag connected (either {@link android.nfc.NdefTag} or {@link android.nfc.Tag}). Note that {@link android.nfc.NdefTagConnection} extends {@link android.nfc.RawTagConnection}.

{@link android.nfc.NdefMessage}

Represents an NDEF data message, which is the standard format in which "records" carrying data are transmitted between devices and tags. Your application can receive these messages from an {@link android.nfc.NdefTagConnection}.

{@link android.nfc.NdefRecord}

Represents a record, which is delivered in a {@link android.nfc.NdefMessage} and describes the type of data being shared and carries the data itself.

Note: Not all Android-powered devices provide NFC functionality.