Powerbase has a powerful import tool and we have tips sheets that cover many of the different angles to importing.
Before you begin
Every import starts with a spreadsheet. And, before you try to import that spreadsheet, you will most likely want to massage that data. Often it's easier to fix errors in your spread sheet before you import rather then trying to clean up the mess afterwards.
Contacts
No matter what you are importing (event participation, contributions etc), you always need to start by importing the records as contacts. Otherwise, there is no contact to assign a contribution or an event record.
We have a screen cast on importing contacts and the CiviCRM handbook has a good chapter on importing.
Event participants
If you want to import a group of contacts and record the event they attended - there are a few ways to do it. Our tip sheet on importing participants breaks it all down for you.
Contributions
Please start with our overview of importing contributions where you will find links to specific pages depending on where you are importing from (Classy, Facebook, Act Blue, etc).
Other data to import
But wait, there's more! You may want to import door knocking activities or, really, any kind of activity. See our tip sheet on importing activities for the full run down.