The general filesystem in Ubuntu is arranged in a single tree with everything under /. This tag should be used for questions dealing with Ubuntu's layout of files, permissions, and files that reside in special locations on disk. Questions related to a particular filesystem should instead use the specific tag for that (for instance: ext4, btrfs, zfs).
In Ubuntu, as in other Linux systems, files are arranged in a single tree with everything under / : the root directory.
A useful reference is the Linux Filesystem Hierarchy Standard, which sets out requirements and guidelines for Unix-like file systems in detail.
Some examples: the kernel and bootloader files are found in /boot, system executables are usually in /bin /usr and /sbin, configuration files are mostly found in /etc, variable files, such as system logs, are in /var, and normal users' home directories are in /home.
A key attribute of Linux file systems is the control of permissions for each file in the system. This stems from the design of Unix systems as inherently multi-user. Files are always owned by some user and some group, and the ability to read, change or "execute" a file are separately defined for the owner, the group, and everyone else. Normal users own the files in their home directories, while the majority of other files are owned by root. In Ubuntu, root normally has no password, and normal users gain permission to modify files owned by other users, including root, with the use of sudo.
A simplified graphical summary:
