Financial Types should be set up before using CiviCRM for financial transactions. If your organization has an accountant and / or a bookkeeper, you'll want them involved in this process.
For those who are familiar with CiviCRM versions prior to 4.3, the old idea of Contribution Types has been enhanced and given a few new names to reflect that CiviCRM now records other sorts of financial information like expenses (grants, bank charges, cost of premiums given away), and is more enterprise-ready in how it tracks money going into and out of bank accounts, accounts receivable, and accounts payable. In an effort to keep bookkeeping complexity and details hidden from CiviCRM administrators who don't need to know about it - fundraisers, events managers, online organizers - version 4.3 introduces the notion of Financial Types.
Financial types let users indicate all the bookkeeping accounts that might be used in the different stages in the lifecycle of a donation or purchase of a membership. So an end user might say they want to buy 2 tickets but pay later. Bookkeepers would record this as accounts receivable - money owed to your organization that is not yet received - as well as revenue for Events. Then later when the check arrives in the mail the payment is recorded as deposited into your bank account, which involves shifting the cost of the tickets from accounts receivable to the bank account. Suppose one ticket is no longer needed, and a refund needs to be made: this involves shifting funds from the bank account to accounts payable - money that the organization has to pay. Rather than having the staff person involved in each of these transactions select the correct accounts, financial types let them all be specified once. First you create a Financial Type for something like Event Tickets, or if your organization has different accounts for your annual fundraising dinner and the courses put on, perhaps one for each. After creating these Financial Types, you will need to set up relationships between them and the Financial Accounts used for various purposes, as illustrated above.
If your needs are simple, you may be able to get by with the default financial types created when you install CiviCRM, since they provide reasonable defaults for related accounts for a simple set of accounts. They also provide good examples to copy and modify if you need to make different ones to meet your organization's needs.
You set up Financial Types at Administer > CiviContribute > Financial Type. Once you have saved a new Financial Type, click the Accounts link beside it at Administer > CiviContribute > Financial Type. Here is where you indicate which Financial Accounts are used for what purpose when the Financial Type is used in the system.