"идти" - it means "to go", "to walk". For example.... You're !walking! down the street. Ти !идешь! по улице. "зайти" - means you are going to a building now. Open the door, step over the threshold. Enter to a school. Зайти в школу.

1. Grammatically, the important thing is that идти has no prefix and is imperfective. Зайти has a prefix and is perfective (its imperfective pair is заходить). 2. Talking about the meaning, идти represents a kind of movement in general with no specification. Зайти means to come over for a short period of time and then usually continue your way to the main destination. Example: - Ты идёшь в супермаркет? Are you going to a supermarket? - Да. Yes - Зайди по дороге в аптеку и купи аспирин. Come over to the pharmacy on your way and buy some aspirin.

идти -передвига́ться, де́лая шаги́.... По улице идёт человек. зайти- нанести визит...................... Зайти к другу в гости.

"Идти" means "to go" or "to walk". The difference between these two words is that if you are going to a structure or things like that you better use "зайти"!