Some very basic descriptions: 

Question: Who is a contact? 

Answer: A Contact is any individual or influencer associated with an account that you want to track in salesforce.

Question: What is an account? 
Answer: An Account is an organization, individual or company involved with your business such as customers, competitors and partners that you wish to track in Salesforce.

Question: What are Leads? 
Answer: Leads are people who are interested in your product and service. Converting leads to loyal customers will provide success within a business. By managing your leads in a systematic and structured way, you can increase both the numbers of leads you generate and how many leads you convert.