The Leads search offers a number of different search fields for filtering your results. You can use as many fields as you like to create hyper-granular searches with very specifically defined results. Or you can keep your search broad by entering just a character or two in one or two fields and sorting through those results.
If, however, you have a large number of leads, it is recommended that you add as much information as you can so the system can narrow down your results and make it easier to find what you need.