The Macintosh SE/30 was the successor to the Apple Macintosh SE and the Macintosh Plus. It ran under Mac OS 6.0.3 or A/UX (Apple’s Unix variant).
Apple had been naming all computers using the 68030 processers with an "x", such as the IIx and IIcx. When it came time to put the 68030 processor into the SE series, they prudently decided that calling it the Macintosh "SEx" wasn't the best marketing idea, so they settled on "SE 30".
Except for the 68030, one of its main differences with the old Mac SE was its separate video RAM (64 KB which could be accessed without slowing down CPU access to the RAM). Its sound system was a limited version of the Apple II GS sound system.
Although the SE-30 had a gray-scale monitor, the onboard video actually supported color. The cost of incorporating a 9" color CRT would have made the machine far too pricey, resulting in the the grayscale CRT. However, the idea of a color CRT would be taken up again with the Colour Classic.
The SE-30 also had an expansion slot that could support a variety of 3rd party cards, an example being a combination 10BaseT Ethernet / color video out card.
Some information provided by Andrew Granger:
The SE/30 could address up to 128MB of RAM, the SE/30 was essentially a Mac IIx/Mac IIcx crammed into a Mac 128/512/Plus/SE-sized case. The one caveat to this maximum RAM configuration is that the ROMs were not "32-bit clean." The ROM implementation was only 24 bits wide, with the result that if you installed any more than 8MB into the SE/30, you wouldn't see any more than 8MB available, but the System Software listing in the "About this Macintosh..." menu item would be *huge* (this same situation happened on the Mac IIx and IIcx also). In order to address more than 8MB of RAM, you needed to either install Connectix's Mode32 extension, or (later) Apple's 32-bit Enabler extension. These extensions patched the ROM code to make the machine 32-bit clean, thus allowing you to have as much RAM as your heart desired, up to 128MB.
Video extension by Luca Rescigno:
SE/30s have been hacked to an even greater extent. There are many different things people have done, but one popular (if rare, and dangerous!) upgrade is to find a Micron XCeed graphics chip. This chip fits into the neck of the CRT, which is a dangerous place to work because of all the built up charge. When installed, it will allow the monochrome 512x384 display to use 256 grays at 640x480! In addition to the 128 MB RAM upgrades some crazy souls have added, others have found out how to install systems beyond 7.5.5 onto 68030 machines. I think the software is called BornAgain, and it lets you run 7.6, 8.0 and 8.1 on old 68030s, although I don't think 68030s can use the HFS+ filesystem introduced with 8.1.