Creating a basic form is as simple as pointing, clicking and setting a few obvious options. Create a Form. Create a Group. Add Elements to the Group. Associate the Group with the Form. Associate the Form with your Joomla menu structure. Done.
But that is just scratching the surface. Lets say you are building a Contact Us form, and want to preset the "Your E-mail" Element if the user is logged in. Just go to that Element in the GUI, select "Eval this text" and enter any valid PHP to fetch the standard Joomla $my->email variable, like this:
return $my->email ? $my->email : "Enter your email address";
Of course, you can do the same for the "Your Name" Element using $my->name.
A slightly more advanced example might be building a custom order query form for VirtueMart orders, so you need a dropdown with the logged on users unfilled orders. This is as simple as adding a 'database join' Element which pulls in the relevant details from the VirtueMart tables. The GUI easily lets you create the necessary database JOIN and WHERE clauses to populate a dropdown with anything you want.
As well as reading SQL data, Fabrik also lets you store submitted form data back into the database, as well as (or instead of) emailing it. You can use this for simple things like keeping a record of all your Contact Us submissions. Or you can use it to control the flow in multiple "chained" form submissions. Or you can get as creative as you want, using the intuitive 'database join' features, and (say) allow users to change their main profile details from a custom form.
Fabrik also lets you define SQL table 'views' in the front end, as well as simple forms. And of course everything can be access controlled using the standard Joomla group features. So it's trivial to build a front end admin interface for viewing, adding and removing table rows.
I've had Fabrik installed for exactly 24 hours, and I have a very professional looking, working custom form for querying the status of VM orders, and a front end admin interface for processing the requests.
And I don't even have the Fabrik documentation. I accidentally used a PayPal e-check, so I have to wait a few days to get access to the site. I'm like a kid waiting for Christmas Day, I can't wait to see what I can do once I have the docs, examples, plugins, etc.
Summary ... yes it helps if you known at least a little about SQL and PHP. But not essential. Get this extension, and invest a few hours in it - you won't regret it!