Tox IDs are strings of 76 hexadecimal characters, generated when a user creates a Tox profile. These IDs are used for adding contacts.
The main part of a Tox ID is called the public key and consists of the first 64 characters.
NoSpam is a small part of a Tox ID consisting of 8 hexadecimal characters that can be changed by the user's Tox client at will to prevent friend request spam.
To understand pros and cons of different ways of sharing Tox IDs, see sharing Tox IDs.