Hi Uthirapathi , UML is a modelling Language which was developed by Rational Rose. It has however become an industry standard now. To design a house for example one needs to know the width and length of the rooms, the size of the doors etc. This is done by an architect by drawing on paper plans. Initially a programmer just writes code. He does not think of things like OOPS, design patterns etc. One should however use these various techniques to create a good program. In large organisations u r given specifications with respect to what is to be done by u. The whole project is designed in a proper manner. One can only undertake planning in software through a design tool. This tool would simplify things to all the developers if they understand it. So that they get the whole picture while programming for only a part of it. This tool used today is UML.
[This message has been edited by rahul_mkar (edited July 18, 2000).]
A few years ago, there were lots of modeling languages. People would argue constantly about which was the best. The three most popular were from Booch, Rumbaugh and Jacobsen. Rational hired these guys one by one and then the three of them came out with UML: The Unified Modeling Language. Now the three guys are called "The Three Amigos". So now there is one really popular modeling language and a few unpopular modeling languages that most folks are giving up on. Thus, UML is probably the only modeling language you need to learn. I think the best book for learning UML is a very small book called UML Distilled, by my favorite author Martin Fowler. See http://www.javaranch.com/bunkhouse.html for the JavaRanch book reviews which include this book.