A brief walkthrough of the most important files
cmdline - The command line issued when starting the kernel.
cpuinfo - Information about the Central Processing Unit.
dma - Contains information about all DMA channels available, and which driver is using it.
filesystems - Contains short information about every single filesystem that the kernel supports.
interrupts - Gives you a brief listing of all IRQ channels, how many interrupts they have seen and what driver is actually using it.
iomem - A brief file containing all IO memory mappings used by different drivers.
ioports - Contains a brief listing of all IO ports used by different drivers.
kcore - Contains a complete memory dump. Do not cat or anything like that, you may freeze your system. Mainly used to debug the system.
kmsg - Contains messages sent by kernel, is not and should not be readable by users since it may contain vital information. Main usage is to debug the system.
ksyms - This contains the kernel symbol table, which is mainly used to debug the kernel.
loadavg - Gives the load average of the system during the last 1, 5 and 15 minutes.
meminfo - Contains information about memory usage on the system.
modules - Contains information about all currently loaded modules in the kernel.
mounts - Symlink to another file in the /proc filesystem which contains information about all mounted filesystems.
partitions - Contains information about all partitions found on all drives in the system.
pci - Gives tons of hardware information about all PCI devices on the system.
swaps - Contains information about all swap partitions mounted.
uptime - Gives you the uptime of the computer since it was last rebooted in seconds.
version - Gives the exact version string of the kernel currently running, including build date and gcc versions etcetera.
And here is a list of the main directories and what you can expect to find in there:
bus - Contains information about all the buses, hardware-wise, such as USB, PCI and ISA buses.
ide - Contains information about all of the IDE buses on systems that has IDE buses.
net - Some basic information and statistics about the different network systems compiled into the system.
scsi - This directory contains information about SCSI buses on SCSI systems.
sys - Contains lots of variables that may be changed, including the /proc/sys/net/ipv4 which will be deeply discussed in this document.
As you can see, there is literally hundreds of files in the /proc filesystem that may be read and checked for information, and we haven't looked at half of them here.