Jenkins monitors executions of repeated jobs, such as building a software
project or jobs run by cron. Among those things, current Jenkins focuses
on the following two jobs:

1. Building/testing software projects continuously, just like
CruiseControl or DamageControl. In a nutshell, Jenkins provides an
easy-to-use so-called continuous integration system, making it
easier for developers to integrate changes to the project, and
making it easier for users to obtain a fresh build. The automated,
continuous build increases the productivity.
2. Monitoring executions of externally-run jobs, such as cron jobs and
procmail jobs, even those that are run on a remote machine. For
example, with cron, all you receive is regular e-mails that capture
the output, and it is up to you to look at them diligently and notice
when it broke. Jenkins keeps those outputs and makes it easy for you
to notice when something is wrong.