Accounts represent your brokerage account. Depending on your broker you can link the Account with your broker using TRADE and send orders from Portfolio123. Accounts can also be "paper" which use play money to simulate a real brokerage account.
Within Accounts, Strategies are used to organize the holdings and transactions. Strategies can be either manually managed, or following a Model. Strategies are a great way to track separate investing ideas within the same account.
A Model represents an investment strategy. Broadly speaking, there are two kind of models based on management style: active or passive. Active models continually reevaluate holdings and replace them with better opportunities if they arise. Passive models usually stick with the same holdings and only adjust the weights. You can begin following a Model by buying the current holdings and following its future recommendations.