Hi, I'm new here, but I have a quick small question. I'm sorry if it sounds stupid, but I'm new to the UNIX world after using VMS (or Windows) my entire life. I my Mac runs Leopard, and I was wondering;
My friend me through Mac's "Terminal" you can create and program your own applications by just using the UNIX codes and just in Terminal. Is this true? If it is not, what kind of program or application would I need to make any kind of application?
Thanks in advance. 
Yes, UNIX was written in C in a terminal to start with.
While UNIX is "native" C at the core, you can develop useful small to medium applications with Unix tools called from inside shell scripts or one line commands - from the terminal. shell == DCL, except what you can do in UNIX is an extreme superset of that because of tools like awk grep and sed. Entire applications are written in shell. There are also a lot of interpreted languages that let you do things like develop modest db apps in shell or performs extended precision calculations <- examples.
DCL is verbose; shell cryptic and terse... It is possible to write one line of code that does what 20 lines of DCL might do. And a lot harder for beginners to read.
The one thing you will have to learn is regular expressions to get the most out of unix tools. Another concept is processes. Unix creates lots of child processes, sometimes each command is a separate child. This affects variable scope.
Your default shell is probably bash - consider googling for 'Advanced bash guide' and see how you fare with it. Assuming you were okay with DCL.
A little advice from someone who owned two OS X-capable Macs and used all the powerful extras he could on both:
OS X Developer Tools, free from Apple (you just have to sign up to their Developer Community, also free) enhances what Terminal can do in terms of compiling and testing programs created with its own resident utilities. Apple's X11 extends those bounds considerably more. If you bought your Mac new or refurb from an authorized Apple reseller, CDs with those packages on them should have come with it. If not, and your Mac happens to burn CDs/DVDs (most do nowadays), then you can download the necessary disk images direct from Apple. Just Google for Developer Tools for Leopard and Mac OS X X11 for Leopard, and look for links where the URLs happen to have either "www.apple.com" or "developer.apple.com" at the beginning.
Twelve years of Mac home use comes in handy sometimes.
BZT