What is the difference between /etc/fstab and /etc/mtab?

/etc/fstab is a list of filesystems to be mounted at boot time. If you want your Windows or file-storage partitions mounted once your computer boots, you'll need to put appropriate entries into /etc/fstab.

/etc/mtab is a list of currently mounted filesystems. If you have a disk connected but not mounted, it won't show up in the /etc/mtab file. Once you mount it, it will show up there.

Note also, that with systemd (to which Ubuntu switched beginning from 15.04 release) it is possible to declare filesystems that need to be mounted at boot via *.mount files. See James Oguya's tutorial on the topic.

For more info, read mount manual.

TL;DR

• /etc/fstab is a created by the user. It contains list of volumes to be mounted by mount.
• /etc/mtab is a created by the system. It contains a list of currently mounted devices.
• The format of the files is similar. After mounting a new device, copy the relevant line from /etc/mtab to /etc/fstab so that it will be auto-mounted after boot or when calling mount -a.

Quotes from the mount manual

The /etc/fstab, /etc/mtab and /proc/mounts files

The file /etc/fstab, may contain lines describing what devices are usually mounted where, using which options.

The programs mount and umount maintain a list of currently mounted filesystems in the file /etc/mtab.

When the proc filesystem is mounted (say at /proc), the files /etc/mtab and /proc/mounts have very similar contents. The former has somewhat more information, such as the mount options used, but is not necessarily up-to-date.

mount -a

mount -a [-t type] [-O optlist]

(usually given in a bootscript) causes all filesystems mentioned in fstab (of the proper type and/or having or not having the proper options) to be mounted as indicated, except for those whose line contains the noauto keyword. Adding the -F option will make mount fork, so that the filesystems are mounted simultaneously.