Lists in SalesforceIQ are comprised of leads, opportunities, deals, and so on. These items can be either individual Contacts or Accounts.
In SalesforceIQ, Contacts are the individual people, or points of contact, a user adds to a List and/or an Account.
By comparison, an Account refers to the Accounts a user adds to a List. Accounts are comprised of all the Contacts in that Account.
For instance, Jim, Pam, Dwight, and the rest of the individuals who work at Dunder Mifflin are Contacts. Dunder Mifflin itself is the Account.
Video: Accounts vs. Contacts (~1.5 min)
See the following articles to learn more about working with Contacts: