Explaining the different series and styles
 

I am new to the Audi world and trying to figure out just what I have so I can order the correct parts, manuals and so on. The car did not come with any manuals, owners manual, repair manual, nothing.
I have a 2003 Audi A4 convertible with a 1.8 turbo charged engine and an automatic transmission.
I see cabriolet columns but when I click on them my year is not an option.
I see convertible associated with the letters TT. How do I know if I have a B6 platform or something else.
What years and models share similar parts.
I realize these are very basic questions to some of you owners but like I said, this is my first Audi so thank all of you in advance for any information you might can share with me.

Well I don't feel so bad now because apparently no one else knows either. Maybe the right person will see this and reply. There is a lot of information here to be covered.

Series/Model/Chassis alphabet soup
 

Being new to the Audi family of enthusiasts, I’ll take a stab at it...Ax is baseline model based on age/size/generation; Sx is sport version which may have chassis/suspension performance options, again model based age/size/generation; TT is high performance coupe. Bx refers to chassis/age/generation.
Example A4 (larger than previous A3 model) Cabriolet (convertible top), B6 (chassis superseded earlier B5, but before longer B7 Avant-hatchback version)...

A4 B6 (2002 ->2005.4) looks like this:
https://cimg6.ibsrv.net/gimg/www.aud...d7a11fcadb.jpg
A4 B7 (2005.5->2008) is an updated version and looks like this:
https://cimg8.ibsrv.net/gimg/www.aud...520e53bc1a.jpg
This is a TT, totally different car:
https://cimg1.ibsrv.net/gimg/www.aud...adbb4c3085.jpg

There is also A3 cabriolet:
https://cimg9.ibsrv.net/gimg/www.aud...cf0461b454.jpg
and A5 cabriolet
https://cimg2.ibsrv.net/gimg/www.aud...6402caef73.jpg

TT and A3 are more related to VW Golf than A4 models. In general, A5 B8 (2 door coupe, 5-door and cabriolet) is pretty much same as A4 B8 (4 door sedan and 5-door wagon) of that time. Their owners might disagree tho. A3 and A5 came on or after 2008 which is the last model year of A4 B7 cabriolet.

Visually B7 interior looks newer, rear end looks quite the same, lights are different. Front end makes most difference as you can see in the pictures. Under the hood many components are the same though.

So in short, depending on what type of spare part you need it can be found in model years between 2002-2008, most certainly between 2002-2004. Also some parts are shared with A4 sedan/wagon too.