Generally speaking the definitions are quite flexible but here is the common definition:
Lead: An unqualified opportunity. This could be as simple as a business card or a conversation overheard in a pub
Contact: The individuals your business interacts with during the normal course of the day (normally clients but also can be suppliers, partners etc.)
Account: The organisations your business interacts with during the normal course of the day (normally clients but also can be suppliers, partners etc.)
Opportunity: The opportunity to sell a service or product. This generally is associated to an account or contact, has a value attached to it and a probability of success.
Leads can be converted into an Account, Contact and Lead. Generally contacts will be associated to an account, the company they work for but this is not compulsory.
Regards,
Leon Tribe