Getting information

pwd	tells what directory you're in
cmd version	what version of cmd is being used?
apropos foo	tells all commands that deal with foo
ls -al	list all files, including hidden files, and display file information
which foo	tells path to binary foo (if foo is in path)
whereis foo	finds files or directories names 'foo'
locate foo	finds directories or files whose names contain 'foo'
find . -type f -print| xargs grep stuff /dir	applies grep to a whole tree (/dir in this case)
find /dir -name filename	finds filename in /dir
more /proc/cpuinfo	in Linux, returns processor type, processor speed, cache size, etc.
more /proc/meminfo	in Linux, returns information on memory
uname	returns operating system
more /etc/issue	in Linux, gives operating system information
hostname	returns name of machine
host -n machine	gives IP address of machine
rusers -l	tells who's on the network
nslookup hsph.harvard.edu	returns IP number
nslookup 134.174.190.44	returns hostname
rwho	similar to rusers
df -l /dir	amount of free disk space in /dir
du -l /dir	amount of used space in /dir
du -h /dir	amount of used space in human-readable form