Relationships are the connections between objects.

People - Can be contacts on Accounts and can have Opportunity Contact Roles on Opportunities. People can also generate activities.
i.e. Frank is a contact on the Engagio account, but is also an Influencer on a new business opportunity that is open for Engagio, and has recently visited the pricing page.

Accounts - Can have people who are contacts, opportunities that are open or closed, and can have activities from people related to the account.
i.e. Engagio is a company that has many different contacts associated to it in the CRM. There is an open new business opportunity for Engagio and there has recently been web visits from 10 people on the account.

Opportunities - Are always associated to an account and can have people with Opportunity Contact Roles on them.
i.e. Engagio - New Business Foundation Opportunity is an opportunity on the Engagio account and has the CMO, Heidi Bullock as a decision maker on the opportunity.

Activities - Are generated by anonymous or known people from Accounts.
i.e. There were 4 website visits from Engagio this past week, 1 from an anonymous person and 3 from the VP of Marketing.

Below is a chart that demonstrates what is possible for each object type: People, Accounts, Opportunities and Activities.

Here is a a more specific example for finding accounts with new business opportunities, where there are VPs who have opportunity contact roles, and have been visiting the pricing page.