Let me break it down for you.
Lead – this is my own definition, Sales leaders in your organization may differ in how they want it to be used (which Salesforce will accommodate). A “lead” should be used for what some organizations call a “suspect.” In other words, we have a name, some contact information (phone, email address, mailing address, etc. – but maybe not all of these), our organization believes this person could potentially, maybe, someday, sort of purchase all or part of our offering. Oh, and we think they have a heartbeat. These “leads” may have come from a purchased list, trade show fishbowl, web form submit or some other miscellaneous source. These are raw and unqualified.
At some point in your interaction with these individuals you will do some sort of qualification and “convert” them. Typically there is a process for this, in its simplest form I coach my clients to convert a lead when they believe that the person you are working with has a “felt need” for your product/service, plans to buy within a specified timeline, and has the ability to decide on the purchase or is a significant influencer. I wouldn’t convert a Lead who some organizations would consider a “scout,” i.e. someone looking on their own for a solution or sent out by someone in leadership to investigate options.
Account – these are pretty straight forward. An Account represents a logical grouping of people, most often a company, division of a company, or non-profit organization. If you are a non-profit using version 3.0 or higher of the Non-profit Starter Pack or NGO Connect this can also represent a “household.” When working with larger organizations that are made up of numerous divisions, locations, or smaller operating companies you have the ability to link Accounts together using the Parent Account field.
Contact – in its simplest form a Contact represents a person associated with an account. If the “Account” or company (or other entity as described above) has already made a purchase you may have additional Contact records not at all associated to the buying cycle. A common use case for having contacts not involved in a purchasing decision would be in a company that provides some sort of service. For example, maybe your firm sells technology which comes with a support contract. The users of the technology who may not have participated in the buying cycle may at some point need to call, email, or live chat with your organizations technical support group. You would want to log this communication and/or support case and should tie that event back to a contact. Multiple Contacts can be and often are associated with an Account.
Opportunity – think of an Opportunity as a financial transaction your company is pursuing with another company or individual. Some organizations may call this by a different name such as a “deal”, “sale” or other term. Multiple Opportunities can be and often are associated with an Account.