Linux certification is for those folks who have worked with Linux on a day-to-day basis. Just as there are many different flavors of Linux, you can expect as many flavors of Linux certification. Do you like vanilla? If so, you might want to consider a vendor-neutral certification from CompTIA, Linux Professional Institute, or Sair.
Red Hat's certification is the leader in vendor-specific Linux titles. To be a Red Hat Certified Engineer, you'll need a day to dedicate to a three-part examination:
A multiple-choice exam
A hands-on server install and network services exam
A hands-on debug lab on a Red Hat system
Of course, this day of testing is on-site at a Red Hat testing facility.