"shell" in Makefile

["Easthope" <peasthope@cablelan.net>]

Debian users,

In 

http://git.kernel.org/?p=linux/kernel/git/linville/wireless-dev.git;a=
summary

Vivek described a makefile containing this text.

obj-m := hello.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
    $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

shell is not a command in the Etch here.

So what is "shell uname -r" and "shell pwd"?
Why not just $(uname -r) and $(pwd)?

Thanks,        ... Peter E.