Cron doesn't 'manage' building of software, it's just a scheduler that will run scripts/programs on a specific time interval indicated by you. Nothing more and nothing less.
I don't know if the role of build engineer is something specific. We include the 'build' part in the role of a Release engineer
. Look at the link provided for a pretty detailed description.
I don't quite understand what you mean with your last question but I'm assuming you're referring to creating jobs in Jenkins. Jenkins is a continuous integration software that allows you to create jobs to build software for example. Have a look at their wiki
for a complete description.
Last but not least, try to use a more descriptive title next time. Your title is only remotely descriptive of your questions.