A Linux "Distribution" represents a somewhat integrated installation package that typically provides such components as:
Utilities to help set things up to install Linux. Since Linux can run (even just in the IA-32 "world") on a wide variety of hardware of varied configuration, there are a number of ways to install it.
This typically includes boot options such as:
Booting from floppy, or
Booting by running an MS-DOS utility called LINLOAD.EXE that loads a Linux kernel from the hard disk or CD-ROM,
An MS-DOS partition on a local disk,
A remote computer using FTP (File Transfer Protocol),
A remote computer using NFS.
Various software organized into packages, along with utilities used to install,deinstall and upgrade the software. More modern distributions have increasingly sophisticated tools for these purposes that allow software to be safely upgrade, almost always with no need to reboot anything, use cryptographic protocols to validate that nobody has "hacked" with the software, allow the binary package to be rebuilt from original source code, and enforce dependancies between packages (you can't very well use a C library without a C compiler).
A set of "base" packages is required to have a minimal functioning system, normally including the kernel, system libraries, and some file and network utilities. It is possible to run a really stripped-down system off a single floppy. But usually the default "base" includes quite a lot of packages so that you get a system that can do quite a lot of useful things.
Linux distributions typically come with a wide variety of additional packages not all of which will be of interest to any one person in particular. Packages included include such things as:
A bunch of games...
Programming utilities including source code management systems, profilers, debuggers, lots of libraries...
Compiler construction tools including as flex, yacc, bison, just to name a few...
Numerical analysis tools including some Matlab "clones," statistical software, graphing packages...
Lots, lots, lots, lots more. Common distributions have hundreds of packages available; it's not uncommon for vendors to sell distributions that have multiple CDs containing supplementary packages. Debian offers upwards of 8000, that's eight thousand packages.
There are typically configuration utilities to assist novices through the complexities of configuring things like network services and the X Window system. They're also helpful to the "expert" as they cut the time and mental effort required, and let you get to using the system more quickly. I'd rather use Debian's pppconfig to configure my PPP connection than fight with config files myself, given a choice.
I first installed Linux from a 1993 version of the Infomagic Linux Developers CDROM set, using the Slackware distribution. In theold days, there were only 2 CDs in the set. Things sure have changed since then.
These days, I am mostly running Debian.
There are a whole lot of distributions, including a lot of variations/relations to one another.