RedHat is the company that created RedHatLinux. Founded in 1993, they were a major force in transforming Linux from an enthusiast occupation to a commercially supported and trusted OperatingSystem – while contributing most of their work back to the community. They offer several certifications: RedHat Certified Technician, RedHatCertifiedEngineer, and RedHat Certified Architect. They also run the community-driven FedoraProject, a clone of RedHatLinux which is not commercially supported.
rpm(8) is the RPM Package Manager (though the first "R" used to stand for RedHat). A good source for up-to-date RPM files is FreshRpms (also has a howto on using apt(8) with RPMs). http://rpmfind.net and http://rpmseek.com are also very useful resources.