It really depends on the distro. Linux can literally be customized at any level and even the most basic components, such as the filesystem or cron scheduler, can be swapped out with alternatives.
Generally, the biggest difference between distros is that some come largely pre setup with all necessities taken care of out of the box with an easy install, such as Ubuntu, whereas others allow users to configure their operating system to the smallest detail and only provide the tools to help them do so, such as Gentoo. Generally the latter distros require more savy users and will require them to familiarize themselves with the more mundane differences between distros.
I think the most defining characteristic of each distro is usually the package manager or equivalent system. Also, the amount and quality of packages available for distro. Most major distros have their own and each determines a lot about the distro. Debian is known for apt-get for example (which Ubuntu borrowed) which is held in high regard for being relatively easy to use. Gentoo, however, uses portage, which is slow because it usually downloads packages as source and compiles them before installation, but provides a ridiculous amount of flexibility in configuring a system, right down to specifying the compiler flags packaged will use. Both of the these have large repositories of packages for users to browse and select from. Slackware, by contrast, has no package management system to speak of, but does have packages which you can download and install separately.
Generally, the biggest difference between distros is that some come largely pre setup with all necessities taken care of out of the box with an easy install, such as Ubuntu, whereas others allow users to configure their operating system to the smallest detail and only provide the tools to help them do so, such as Gentoo. Generally the latter distros require more savy users and will require them to familiarize themselves with the more mundane differences between distros.
I think the most defining characteristic of each distro is usually the package manager or equivalent system. Also, the amount and quality of packages available for distro. Most major distros have their own and each determines a lot about the distro. Debian is known for apt-get for example (which Ubuntu borrowed) which is held in high regard for being relatively easy to use. Gentoo, however, uses portage, which is slow because it usually downloads packages as source and compiles them before installation, but provides a ridiculous amount of flexibility in configuring a system, right down to specifying the compiler flags packaged will use. Both of the these have large repositories of packages for users to browse and select from. Slackware, by contrast, has no package management system to speak of, but does have packages which you can download and install separately.