UNIX / Linux:
That’s a very broad question and could be answered any number of ways. Probably the simplest answer is that from a technical point of view there are no major differences. Most people aren’t satisfied with believing that Linux and UNIX are very similar though. Here’s a list of the most obvious remaining differences.
Origin: UNIX originated in the laboratories of universities and large corporations as an initiative within the context of those organisations. Linux was begun by a university student (Linus Torvalds) without any initial support from any large organisation. Linux also began as in mimickry of other well-known UNIX-like implementations whereas the initial UNIX implementations were original research. Most commercial UNIX versions are also derived from that early research.
Service Model: Most UNIX versions operate on the basis that you can buy help (support and service contracts). Although such things are increasingly available to Linux technologists traditional arrangements consist of providing your own help with the assistance of a community of like-minded people. Linux is big and access to communities is more important than say it is for IBM mainframes.
Equipment: Although Linux runs on many kinds of equipment it is best known for its support of commodity IBM-Intel PC-based hardware. Most of the more popular UNIX flavours focus on high-performance hardware usually of a proprietory nature or using high-end standard computing architectures like SPRAC. With ever-increasing gains in pc-hardwarelike Serial-ATA this distinction is not as large as it used to be especially for low performance uses like desktops.
Licensing: Linux follows the Free Software Foundation’s radical licensing model which provides a great deal of liberty to those that interact with Linux technology. UNIX versions provided by other vendors have profit strategies embedded in them. People who offer Linux services might have a profit strategy but Linux itself doesn’t. This means that vendor lock-in is less of an issue with Linux than it is with other UNIX offerings. It also means that organisations big enough to have a center of computing competancy always have the choice of doing it themselves.
Honesty: Linux and related software is extremely visible. You can find out about flaws before yocommit to the technology rather than afterwards. Because of this a version number in Linux is a more relable indicator of the quality of the software than in UNIX. For example most Linux software spends a long time being version 0 (zero) before it ever qualifies for the label version 1.
In terms of quality performance and feature set there’s little to separate Linux from the other UNIXes. Linux has yet to provide genuine real-time scheduling which some other UNIX versions do well.