A focus is just another way to satisfy the Material (M) component needed to cast a spell where it's required. If a spell needs a material component, you can use your focus instead. If the material component has a monetary value, for example a Pearl worth at least 100gp, then that specific material is still needed.

Can you wear a spellcasting focus?

Yes. You need to hold the arcane focus for it to substitute as a material component.

Is a spellcasting focus consumed?

If a spell's material components are consumed, can a spellcasting focus still be used in place of the consumed component? Nope. A spellcasting focus can be used in place of a material component only if that component has no cost noted in the spell's description and if that component isn't consumed.