A lead is the term we use for each instance that a customer/potential customer reacts to product that they have not previously reacted to.
When a prospect calls your company, regarding products they have seen advertised, it can be entered as a new lead (assuming that this is the first time they have seen a particular advert, and it is why they have called). This allows you to quickly see all communications that derived from any lead on the system.
If a prospect calls as part of an ongoing communication to do with a previously viewed advert then each contact would be logged as an event against the existing lead.
Typical questions that can be set alongside leads are:
When adding a new lead to the system you can specify the interest (product/service) that the prospect contacted your company about.