Tanwin as a general name is the -n sound at the end of the word. It can come in different flavors: -an, -in, -un.
The (-an) Tanwin is the only one that would come with Alif, unlike the others. For some linguistic and speech-related issues within the language, this Tanwin should be written with Alif. However, there are few exceptions (like if the word ends with Ta-Marbúta; No Alif is added then with the Tanwin).