'HDTV" receiver is a pretty generic term. It could mean the special HD cable box from your cable company (which they should/will provide.)
It can also mean an over-the-air (OTA) receiver will can let you pick up HDTV with your antenna, the same way you might do with a 'normal' Tv signal from an antenna (the antenn's are the same, the same one for both normal Tv and HDTV.) (HD over the aur looks PERFECT, you either have a perfect picture or you have none; there's none of the goofy signal quality isssues you have with analog/normal Tv.)
It can also mean the HD Sat decoder box your Sat service provides. Most of the big Sat sevice now have HDTV services, some with PVR's. Voom is a new Sat service that offers something like 45 HDTV channels, the most of anyone.
The 'stretch' effect you sometimes see can come from several places. The GIANT HDTV sets are CRT based. The picture can burn in to these so some Tv stations broadcast their 'normal'/analog Tv signal over HD in stretch mode so the black/grey 'bars' on the side of the set woun't show burn in. The people that run those stations are idiots.
All HD sets will have a menu/remote option to strecth the picture yourself, then you can decide to stretch the picture or let it remain in it's 4:3 mode. True HD programs will always come in 16:9 format, so it should fill your 16:9 screen completely.
You can find out loads more at avsforum.