In Italian, consonants can be divided into pure and impure sounds (or simple and complex sounds). While both types can occur anywhere in a word, they only really matter, grammatically speaking, at the beginning of masculine nouns.
Italian vowels are divided into two categories: hard and soft. Hard vowels (A, O, U) cause the consonant that precedes them to be pronounced with a hard sound, while soft vowels (E, I) are preceded by a soft sound.
The distinction between tu and Lei is one of the most confounding aspects of Italian, and one of the most basic. The influence it has on verb conjugations and pronouns is considerable, but more than that, the choice of tu or Lei is a matter of etiquette.