While great deal of webapp look and feel can be accomplished with plain HTML, CSS, and PhotoShop, fancy stuff must be done using DHTML.  There are many cross-browser DHTML libraries for 4th generation browsers (i.e. IE 4, Netscape 4.x), but I don't recommend them because a) 4th generation browsers make up only about 6% of all browsers, and b) they are no longer being maintained.  While 5th generation DHTML libraries support only 5.x or better browsers (i.e. IE 5, Mozilla), there are less tricky workarounds and gotchas.

At this time, there are very few actively maintained 5th generation DHTML libraries and most of them are not complete and lack sufficient documentation.  The one I recommend is DomAPI.  DomAPI 3.01 was just recently released so go take a look.