What kind of a verb is "to be"?
Verbs in English can be sorted in many categories using different criteria.
Verb to be can be classified as:
an irregular verb because its Past Simple and Past Participle forms do not end in "-ed"
either a main verb (for example in a sentence He is a teacher) or an auxiliary verb (e.g. John is writing a letter)
an intransitive verb it does not have a direct object and therfore it cannot be changed to Passive Voice
And probably there can be more categories, but now I can't think of any more.