I hope that I wasn't bump into wrong section in asking this type question..if so..please forgive me.As I mention above, maybe someone can explain to me and give some example about UML diagram.What it is, how to use it, and much more.And i believe that UML Diagram is part of Web Programming (since alot my senior told me that). Thanks in advance ya guys!

It is used to plan the initial design of your program..
Well the only UML diagram ive done so far are object oriented based...
A box for a class, within it there are list of methods and variables.. stating if they are private or public, etc...
and how those classes are linked with each other, using arrows to indicate inheritance or etc...

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UML basically allows you to have a higher level of abstraction of a system than to just write it in a programming language, because it is a "modeling" language. So, its just to create an abstract model of a system and not the actual system itself.

Basically like tm89lee said, its used to define and illustrate associations, inheritance, etc. Basically there are various diagrams that you use for a UML model - state diagrams, activity diagrams, Use-Case diagrams, composite structure diagrams, sequence diagrams, etc. depending on what you intend to model or depict. Its just a modeling language for you to define the behaviour, associations, etc. of components, etc. within a system.

You'd also need to be rather knowledgeable in UML before you start using it, or you model would not depict the actual system correctly. This can happen, and it does happen rather frequently. Sometimes, changes in codes due to errors is not reflected in the model (because you created the model first) if you do not concurrently update the model and the codebase, so your system documentation could end up being completely useless. And not to mention using UML takes up time, a lot of time because you need to draw it (even with UML tools).

As far as I can see, using UML for web programming (assuming you're just developing a web site and not some interactive web-based application). UML is used almost exclusively I'd say in the real world for software engineering to model systems using OOP, and not for general web development/programming - at least, I've not heard of anyone using UML for web works prior to this.

Then there is xtUML which is Executable UML - where you can use a compiler to execute your models. It allows an even higher level of abstraction, and its very useful when it comes to RAD programming works. Did some UML for systems analysis and design; and exclusively xtUML for software analysis and design back in uni if I can recall correctly.

So, if you are just doing web programming, skip UML unless you intend to foray into the world of "real" OOP and application development. Just my opinion.