Accounts, Contacts, Leads & Opportunities suitecrm

Hello
I have a website and IT products development company. I am using CRM. Could you explain the use of Accounts, Contacts, Leads & Opportunities ? Who will be what?

Lets say I have got a call or email from a company, ABB. I am talking with them for a work proposal. What will be that company ABB?

Now that company agreed to work with my company. What will be that company then?

Any more information will be appreciated

Read this (except part about sub-accounts):

And this