In Spain, tú is the default far more often than English speakers expect: between colleagues, often with your boss, with shop assistants your own age. Usted marks respect and distance — older people, a first formal contact, a senior client, officials, formal letters.

• Usted takes the él / ella form: ¿Usted puede…? Ustedes takes the ellos form; vosotros is the informal plural.
• Its pronouns are le / lo / la, and its possessive is su: ¿Le ayudo? — ¿Es su despacho? — Is this your office?
• Don't mix the two: ¿Usted puedes…? is a classic slip.

Switching to tú is called tutear. Someone may offer it — Puedes tutearme or ¿Nos tuteamos? — and from then on, tú it is. Many Spanish offices also pair a first name with usted: Helena, ¿usted qué opina?