iTunes Plus is the moniker for high-end (256 Kbps AAC), DRM-free tracks. Several years ago, Apple stopped serving the traditional 192 Kbps songs that were locked down using FairPlay (Apple's DRM schema). Apple not only began releasing all new content using this new system, but also replacing the old content. So, for marketing sake, they had to give it a name (to differentiate between the old 192 Kbps songs and the new DRM-free 256 Kbps): enter iTunes Plus.
As is the Apple way, to bolster its use and cement the new order, it offered to replace all your old content with these new tracks (for a nominal fee). This meant customers could eventually remove all their old DRM, low quality tracks over time.
iTunes Match is a new service designed to move all your music to iCloud, so it can be shared among all your devices seamlessly. It works by "matching" your current content with the stuff available in the iTunes Store, wether or not your purchased it from Apple. For example, if you ripped a CD and the songs are available in iTunes, Apple will replace it with their version for you. If its not available, your songs get uploaded and stores for future use.
The songs it replaces them with are all high-end, DRM-free, 256 Kbps (so iTunes Plus).
iTunes Plus is the mark of the song's quality, while iTunes Match is a new service meant to store all your music in the cloud. They are two very different things and in no way linked.