All I know is HTML and CSS

This is no problem. Flex a built in component library called MXML. If your thinking in terms of HTML, a <p> tag is equivelent to a <mx:Text /> or a <mx:Label /> tag. Knowing MXML will only get you so far. Actionscript 3.0 is a necessity to for any Flex developer. There are great books out there to get started with Actionscript 3.0. For creating simple applications, you don't need to be an Actionscript guru, but knowing simple events and functions will get you far. Learning Actionscript 3.0 is a great book to get you started.

HTML	MXML
<p>	<mx:Text /> , <mx:Label />
<input type='button' />	<mx:Button />
<div>	<mx:Canvas> , <mx:HBox> , <mx:VBox>
<table>	<mx:DataGrid>
<input type='text'>	<mx:TextInput>
<form>	<mx:Form> , <mx:FormItem label=''>

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I'm comfortable with Flash, how is Flex different?

Flex is basically Flash but for coding. There are no visual elements to make, no instance names to type out, and certainly no text boxes. Flex language is 100% Actionscript 3.0. Even MXML components are compiled as Actionscript when exported. There are some major differences that Flash developers must know when transitioning over. Also remember that MXML is based exclusively off of XML. Each tag must be closed, and must have a root tag.

• Instance names are the same thing as IDs in MXML. ex: <mx:Button id='myBtn' />. myBtn would be the instance name.
• Importing Classes in Flex are much easier. Instead of linking a document class, you can make as many classes as you want. To import a class, you type import mx.controls.Alert
• Coding in Actionscript in Flash, you could either create a class object, or write code on the timeline. In Flex, in your main page, you type <mx:Script></mx:Script>.