Fixed broadband services that require hardware installation by qualified technicians will almost certainly have minimum-term contracts, after which you will be able to 'shop around'.
Service offerings provided under a contract will generally be significantly cheaper, and will have more inclusions and options such as anti-virus program subscriptions. Contracts will mostly be for 12 or 24 months and will specify the arrangements – including costs – involved if early termination is requested.
It is important to check the total dollar amount you are committing to (the 'total minimum charge' must be displayed in every price list), and to be sure you understand and are comfortable with the exit arrangements.
Most service providers will offer 'no contract' or 'month-to-month' broadband deals for those who want the flexibility to either terminate their service without cost or change readily to another service provider (e.g. if they are a tenant with a short term lease, are unhappy with the current provider or believe they can get a better deal).
Mobile broadband services can be prepaid or post-paid (like mobile phones); post-paid or contract services are almost always less expensive in the medium to long term. Prepaid has the advantage that you are not committed to a term contract and you don't run the risk of incurring excess data charges.
The 'per gigabyte' cost of data is usually much higher with mobile broadband than it is for fixed services, and the caps on 'included data' are much lower.