alexban,
Here's the short version of how it works. A SID or DP is simply a coded departure route to get you from the departure runway to a fix some distance away in the aproximate direction you wish to go. Upon reaching this transition fix, (think of it as a place to exit the procedure) you can then proceed along your desired/filed route. Like STARS, they are designed as part of an overall plan to move traffic to and from busy airports in as efficient manner as possible considering many factors including terrain and traffic to/from other airports. Some procedures are RNAV only and may require RNP-2 rated NAV gear.
Lets say for example I am filing a flight plan from Dallas, TX (KDAL) to Van Nuys, CA. (KVNY) Looking at my planning chart, I decide the best route today will be via CNX-DRK-HEC-PMD and the LYNXX7 arrival. Looking at the SIDs (DPs) out of KDAL, I note that the WORTH4 departure has several transitions available. One of them happens to be CNX (a fix I want to cross anyway in order to miss restricted airspace) which is about 400nm W of KDAL. So now I will log into DUATS or call FSS and file this route from KDAL-KVNY. WORTH4.CNX-DRK-HEC-PMD.LYNXX7. Provide this route to the FO and it will be entered in the box and stored as soon as the clearance is received from ATC. Now all we need is PAX and we can do what we do.
Here is a link to the KDAL procedures from NACO.
http://www.naco.faa.gov/digital_tpp_...ADOnly=&page=2
Hope this helps a little.
Best regards,
Westhawk