What is the difference between express and tacit consent?

Express consent is when you actively go for consent.

Tacit consent is when you passively accept consent.

Added: To be more precise with the definition:

Express consent is when you actively agree (either by voice or in writing) that you are in accord with a certain course of action.

Tacit consent is assumed when you don't actually state your agreement, but raise no objection (in voice or in writing) to a certain course of action (i.e.; by standing 'mute).