Is Unix totally different from Linux?

Is Unix totally different from Linux? if not what are the differences except Kernel architecture? Can I develope apps which run on both Linux and Unix? (Do not think I am talking about cross platform compatibility and Java.)
Should I go for Linux Administeration or Solaris Administeratrion?
Some one told me Solaris is based on Unix and I think it is right.
I am just a newbie studying OS.

Well,there isn't one Unix,actually Unix is not a OS (well it was 30 years ago,but I doubt that any Unix like OS now has the original code),its more a type of OS.Linux is often called a Unix-clone,because most of the commands and everything is similar.
If you use the C/C++ language,I think you can port it to the other Unices,since alot of system commands are similar (not sure though,I dont have an Unix installed on my computer).

Heck, Unix is totally different from Unix . (see the Unix Family Tree.

Officially, an operating system isn't a true UNIX unless it has been certified as such and the name licensed by The Open Group. Generally this is what sets apart commercial Unix distributions from the open source efforts such as FreeBSD, NetBSD, OpenBSD, and even Linux (although Linux is a "new" implementation, without any of the original codebase). But honestly most serious developers don't worry about the UNIX license so much as following the Unix standards.

The most common standards system is POSIX compliance. Although this is not strictly limited to Unix systems, Unix is where POSIX is most commonly recognized.

If this all sounds confusing, that's because it is. So much so that there has even been a book written called UNIX, POSIX and Open Systems: The Open Standards Puzzle

Anyway, in spite of all this confusion, there have been certain standards implemented that work well across most modern Unix-ish systems. C programming is one example, also scripting languages such as Perl, shell scripting with BASH or other shells, applications such as Apache, MySQL, PostgreSQL, etc... All these tend to behave predictably accross Unix systems. Also, there is GUI programming using the X-window system (this is a little less predictable), and various desktops, such as KDE, etc... So it is really not that hard to do cross-Unix programming.

If you feel you need a certification, Solaris certification of course has more attraction for large companies, because Sun is so well-recongnized. The most popular Linux system (RedHat) tends not to follow traditional Unix system configuration as closely as Solaris, FreeBSD, or even Slackware Linux. Thus a certification in RedHat probably won't translate well to administrating other systems, while a cert in Solaris is benefical to understanding Unix in general. But, the IT industry is changing. Sun, Oracle, and IBM are each recognizing the potential of Linux, so Linux certification will become more profitable in the future. Solaris, on the other hand, might gradually become used less (only on the Big Computers). Thus Solaris jobs might pay well, but they might become less plentiful.