Customer not interested

Set the Lead as Done. 

Customer is interested

If the Lead (unqualified deal) is qualified enough to become a qualified deal (according to your criteria) you can convert the Lead to a qualified deal.

When converting a Lead the qualified deal get all the info from the Lead (Account, Contact, Product group, User). If you have connected a specific Sales process to the product group you also get a suggested sales process to apply.

By converting Leads to Opportunities you get tracking of how you convert Leads.

You can also convert a lead directly to an order. This is recommended if you are working with short sales cycles.