Back in the early days of Linux, things were exponentially more complicated. The distributions were far less mature, but they also required the use of a particular system account to get certain things done. That account was root, and with it, you had unlimited power over your operating system.
To demonstrate the power of the root account, one trick you could always play on unsuspecting users was to tell them to change to the root user with the su command and then have them issue the following:
rm -rf /
The rm command is used to delete files and folders. In conjunction with r (recursive) and f (force), you would delete everything from the root folder (/), thus rendering your system unusable.
Back then, any command that required administrative privileges was run via the root user. In order to do that, you either had to change to the root user (with the su command) or log in as the root user. Both of these options were eventually considered a security issue. Why? If you logged in as the root user and walked away from your system, anyone could do anything they wanted to it. The same thing holds true with changing to the root user and leaving a terminal window open.
Of course, it's much more complicated than that. Having access to the root user meant if a hacker gained access to your system, they could then change to the root user and wreck havoc on the machine.
Eventually, it was decided something had to give. Out of that need, sudo was born. Sudo stands for "superuser do" and effectively gives a regular user (one that belongs to the admin group) access to administrator-like powers. This solved two problems:
The root user could be deactivated (so it couldn't be as easily leveraged).
Only users in the admin group (more on this in a bit) could run administrative tasks.
This was a significant step forward for Linux, one that not only bolstered the security of the system but also made it easier for users.
With sudo in place, users no longer had to change to the root user or log into that account to run administrative commands (such as installing software). Users could run those admin activities through sudo with the same effect as if they were run from the root user account. On top of that, it offered better control over who could do what on any given system. When attempting to run a command that required admin privileges, a user only had to type their user password (also called their sudo password) and the command would go off without a hitch (so long as it was run properly).
For example, instead of having to first change to the root user with su and then issuing the update/upgrade commands on a Ubuntu-based distribution, you could simply issue the commands:
sudo apt-get update
sudo apt-get upgrade -y
By running apt-get through sudo, the user is granted temporary admin privileges and can successfully issue those commands.
What about users not in the admin group?
With regards to the basics of using sudo, any user that you want to grant access to that particular power has to be a member of the admin group for that distribution. For example, on Ubuntu-based distributions, that group is sudo, whereas, on Red Hat-based distributions, that group is called wheel.
Sudo has made Linux not only more secure but also more user-friendly. No longer having to change to (or log into) the root user account avoids a number of serious security pitfalls and allows you to manage user access to admin tasks.