[Diesel_Dave]

mechman is correct. A DPF is for particualtes, while SCR is for NOx.

Here's my attempt at a quick summary. Diesel emissions mainly come down to NOx and particulates (PM).

PM can be controlled somewhat by changes to the engine itself, but in order to meet modern on-highway regs, it's almost always necessary to have a DPF to clean up the PM after it comes out of the engine.

NOx can be controlled on-engine by using EGR, which can get the levels low, but only to a point. If EGR alone isn't good enough, then the NOx has to be taken care of in the tailpipe either by SCR or a NOx adsorber (aka lean NOx trap). The SCR is extremely effective (95%+ reduction in NOx), but it requires the use of urea, aka diesel exhaust fluid (DEF) and is more expensive and bulkier than a less-effective NOx adsorber.

Different configurations make sense for different applications with different regs. I've seen all the following (for current regs):

1) EGR & DPF
2) SCR only
3) SCR & DPF
4) EGR, SCR, & DPF
5) EGR, NOx adsorber, & DPF

I don't know of any vehicles out there that meet the 2010 North American emissions regs with option 2. I know there are some Euro5 applications that use option 2, but I doubt anybody will make it work for Euro6. Option 1 is getting very difficult to do as well. I think Navistar is the only one using this for 2010+.

Keep in mind that all of these give a different tradeoff between fuel economy, initial cost, & DEF usage. Fluctuating fuel and/or DEF prices also affect which option makes the most sense. For example, SCR gives better fuel economy than EGR, but requires DEF usage--so if fuel is "cheap" EGR may make sense, while high fuel prices make SCR more attractive.