A Web application has no access to hardware, by apparent reasons. So, there is no difference is a printer is connected to LPT1 or in some other way — this is a matter of client's OS, drivers, etc. So, you can only print using a printer formally installed in your system: network, local USB, LPT, whatever; for a Web application, they are nothing but just printer.
Most applications simply rely on the browser functionality and the user's decision to send a page to printer. This non-intrusive way is the best.
Your only concern should be providing printer-friendly content: fluid, contrast enough, free from active controls for user input, and so on. You can generate a special printer-friendly page (the user clicks on the printer icon), but it would be much better to do it through CSS discriminating different media and providing different media-sensitive styles. Please see:
For more advanced stuff, please see this article:
I still recommend the first approach. Your customer knows better when to print and what to print
; your application should respect customer's decision and assist non-intrusively