Here are the options for workflows in SharePoint 2007:

1. Out-of-the-box. These are described at Office Online. They are limited in options but require no additional tools.

2. SharePoint Designer. These are a lot more flexible and created using a wizard interface within SPD. Here's a video tutorial, and MSDN reference, you can search for many more resources.

3. InfoPath Forms Server. The next level up from SPD. Here is a resource to get you started on MSDN. Unlike SPD and out-of-the-box workflows, they are not bound to a specific list and can be redeployed to any list.

4. Visual Studio. The next level again but due to the additional complexity should probably be used only if you don't want to or can't use Forms Server for some reason. Strictly ASP.NET based. Resource from MSDN. As with InfoPath, can be redeployed to any list.

Here's an overview on MSDN of all of the options. You should start there to learn the pros and cons of each and how they work.