All files accessible in Linux, are arranged in one big tree: the file hierarchy, rooted at /. These files can be spread out over several devices. File system refers to the files and directories stored on a computer. A file system can have different formats called file system types. These formats determine how the information is stored as files and directories. Some file system types store redundant copies of the data, while some file system types make hard drive access faster.
Wikipedia: File system

There are several ways to determine which file systems are used in the running operating system.

The fstab (or file systems table) file is a system configuration file commonly found at /etc/fstab on Unix and Unix-like computer systems. In Linux it is part of the util-linux package. The fstab file typically lists all available disk partitions and other types of file systems and data sources that are not necessarily disk-based, and indicates how they are to be initialized or otherwise integrated into the larger file system structure.

To get it, just run the command:

# cat /etc/fstab

The fstab file is read by the mount command, which happens automatically at boot time to determine the overall file system structure, and thereafter when a user executes the mount command to modify that structure.

The mount command attaches a filesystem, located on some device or other, to the file tree. Conversely, the umount command will detach it again.

The standard form of the mount command is:

# mount -t type device dir

This tells the kernel to attach the filesystem found on device (which is of type type) at the directory dir.

The following command lists all mounted filesystems:

# mount

The umount command detaches the specified file system(s) from the file hierarchy. A file system is specified by giving the directory where it has been mounted.

# umount dir

For more information just run command:

# man mount

Another way to find out what file systems are in use at the moment is to call the df command. Call with -T option additionaly print file system type.

# df -T

CentOS 7

# df -T

Filesystem          Type     1K-blocks    Used Available Use% Mounted on
/dev/mapper/cl-root xfs        8374272 4202560   4171712  51% /
devtmpfs            devtmpfs    497496       0    497496   0% /dev
tmpfs               tmpfs       508184       0    508184   0% /dev/shm
tmpfs               tmpfs       508184    7096    501088   2% /run
tmpfs               tmpfs       508184       0    508184   0% /sys/fs/cgroup
/dev/vda1           xfs        1038336  280616    757720  28% /boot
tmpfs               tmpfs       101640       0    101640   0% /run/user/0

tmpfs
tmpfs is the name of a temporary file system used on many Unix-like filesystems. It is mounted and structured like a disk-based filesystem, but resides in volatile memory space, similar to a RAM disk.

This means that all the content in tmpfs is temporary in the sense that it’s not permanently written to the disk and in case the tmpfs is unmounted, the system is rebooted or the power is cut all content will be lost.

Mount options for tmpfs:

# man mount

for the search press / type for tmpfs then press Enter

xfs
xfs is a 64-bit, journaled filesystem developed by Silicon Graphics, Inc. It was the default filesystem in IRIX for many years, and has also been ported to the Linux kernel.

xfs excels at parallel I/O, data consistency, and overall filesystem performance. It is well suited for real-time applications, due to a unique feature which allows it to maintain guaranteed data I/O bandwidth.

Today xfs is supported by most Linux distributions and has now become the default filesystem on RHEL (Red Hat Enterprise Linux), Oracle Linux 7, CentOS 7 and many other distributions. Originally XFS was created to support extremely large filesystems with sizes of up to 16 exabytes and file sizes of up to 8 exabytes.

Mount options for xfs:

# man mount

for the search press / type for xfs then press Enter
Wikipedia: XFS

openSUSE Leap 42.3

> df -T

Filesystem     Type     1K-blocks    Used Available Use% Mounted on
devtmpfs       devtmpfs    500352       0    500352   0% /dev
tmpfs          tmpfs       507652       0    507652   0% /dev/shm
tmpfs          tmpfs       507652    2064    505588   1% /run
tmpfs          tmpfs       507652       0    507652   0% /sys/fs/cgroup
/dev/vda1      btrfs     10005504 4782676   4238188  54% /
/dev/vda1      btrfs     10005504 4782676   4238188  54% /usr/local
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/lib/mariadb
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/lib/machines
/dev/vda1      btrfs     10005504 4782676   4238188  54% /home
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/log
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/lib/libvirt/images
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/spool
/dev/vda1      btrfs     10005504 4782676   4238188  54% /tmp
/dev/vda1      btrfs     10005504 4782676   4238188  54% /opt
/dev/vda1      btrfs     10005504 4782676   4238188  54% /boot/grub2/x86_64-efi
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/opt
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/tmp
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/lib/pgsql
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/lib/mysql
/dev/vda1      btrfs     10005504 4782676   4238188  54% /boot/grub2/i386-pc
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/crash
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/cache
/dev/vda1      btrfs     10005504 4782676   4238188  54% /srv
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/lib/named
/dev/vda1      btrfs     10005504 4782676   4238188  54% /var/lib/mailman
tmpfs          tmpfs       101532       0    101532   0% /run/user/1000

tmpfs
See description in section CentOS 7

btrfs
Btrfs is a copy-on-write filesystem for Linux aimed at implementing advanced features while focusing on fault tolerance, repair, and easy administration.

By default, openSUSE is set up using Btrfs and snapshots for the root partition. Snapshots allow you to easily roll back your system if needed after applying updates, or to back up files.

Mount options for btrfs:

> man mount

for the search press / type for btrfs then press Enter
Wikipedia: Btrfs
openSUSE > Wiki > SDB:BTRFS

Ubuntu 17.04

$ df -T

Filesystem     Type     1K-blocks    Used Available Use% Mounted on
udev           devtmpfs    484032       0    484032   0% /dev
tmpfs          tmpfs       101372    3528     97844   4% /run
/dev/sda1      ext4      10253588 4714784   4998236  49% /
tmpfs          tmpfs       506848       0    506848   0% /dev/shm
tmpfs          tmpfs         5120       4      5116   1% /run/lock
tmpfs          tmpfs       506848       0    506848   0% /sys/fs/cgroup
tmpfs          tmpfs       101368       4    101364   1% /run/user/111
tmpfs          tmpfs       101368       0    101368   0% /run/user/1000

tmpfs
See description in section CentOS 7

ext4
The ext4 filesystem is an advanced level of the ext3 filesystem which incorporates scalability and reliability enhancements for supporting large filesystem.

Mount options for ext4:

$ man mount

for the search press / type for ext4 then press Enter
Wikipedia: Ext4