in css designing when we have to use ID and when class?

This would be better off in the CSS Forum

It's a very simple distinction: class is meant to be used on multiple elements. Id is for unique elements, and should be used no more than once per page.

Also, you may have multiple classes on a single element, but you can have no more than one id per element.

You can also have an ID AND a class on an element.

Or, if you're feeling frisky, an ID and multiple classes . To place multiple classes on an element, just put a space between them like this:

<div class="funny news section" id="awesome">

Now, three sets of rules apply to one element. This brings up another interesting point, involving browser compatibility. Wouldn't you know it that all of the browsers see an element with multiple classes in the correct way, except for IE6 and below? IE6 is fine if you state all of the rules separately like this:
CSS

.funny {
/*rules*/
}
.news {
/*rules*/
}
.section {
/*rules*/
}

but not like this:
CSS

.funny.news.section {
/*rules*/
}

All of the browsers except for IE6 and below see the above rule as belonging to any element with all three of those classes on it. What IE6 sees, instead, is only the first class .funny. This means that now, instead of obeying the specific nature of the selector, all elements with only the single class, "funny" will have this rule. I almost pulled my hair out the first time this happened to me, I was so mad!

I have trouble getting my hands around this one as well. Lets say you have a banner that appears twice on a page, identical both times. It appears twice. That means you should use class?

Header appears only once on every page. That means you should use id.

In the above example, what is the disadvantage to using class for all headers rather than id?

If you only intended to use the banner once per page but screwed up on a page and placed it twice using id I suspect it will work.

I understand when you are supposed to use each one more or less, I just don't understand why? Why not just always use class?

The distinction is more than just how many times you use the element, it also determines specificity.

That helps and thanks!!!!

To clarify, an ID will have the highest specificity on the page?

It shouldn't appear more than once on a page but if it did, say in the case of the banner example, you had a banner using ID appear twice, is that proper coding or should you use class?

It will normally have the highest specificity, unless there is another combination with a higher point total, which would then take precedence. If you have more than one id of the same name on the same page, your code won't validate and it might not display properly in all browsers. If there is a change that it could repeat, that's the time to use a class.

That clears it up a lot.

Thanks again.!!!!!