Differences Between sp and spatstat?

I'm brand-new to R and my specific interest is spatial statistics and the
integration of R and GRASS. I just bought a copy of "Applied Spatial
Analysis with R" and have started reading it.

   I've used GRASS on linux since the mid-1990s (and Microsoft-based GIS
since the mid-1980s) and statistics in my academic and real-world work since
the the early 1970s, so my ignorance is specific to R and spatial statistics
with it. I'm a stream ecologist/fluvial geomorphologist, run my own
consultancy, and focus on the natural resource industries.

   The book references the sp package and I find another called spatstat.
What are the differences between them?

   I've also just signed up to subscribe to the R-help mail list because I
need to learn how to install packages (the instructions in the book for ctv,
and those I found on someone's Web site) aren't working for me.

   While I'll be asking a lot of questions at first I expect to be providing
answers and insights before too long.

Rich