According to google, Conditional tags are Blogger template tags that allow us to discover even greater flexibility and better control over the template design. We call them conditional because they allow us to specify which parts of the template will appear under certain conditions. If in any chance you want to place a certain widget in the certain place in your blog like Home page, Post pages or Static page etc, this might become handy.
The simplest Conditional tags there is as state below (note that these are the old one. It might not work on most recent templates) :~

<MainPage></MainPage>
<ArchivePage></ArchivePage>
<ItemPage></ItemPage>
<MainOrArchivePage></MainOrArchivePage>

If you have something in your template that you only want to appear on the main page of your blog, simply put <MainPage> and </MainPage> tags around it. This can be apply as well for the other page types. More info here.

List of Conditional tags (New conditional tags. Work on most recent templates)

1. Index pages (include homepage, labels and archive page)

<b:if cond='data:blog.pageType == "index"'>
..execute if satisfy the condition
</b:if>

2. Post (item) page

<b:if cond='data:blog.pageType == "item"'>
..execute if satisfy the condition
</b:if>

3. Static Page

<b:if cond='data:blog.pageType == "static_page"'>
..execute if satisfy the condition
</b:if> 

4. Archive page

<b:if cond='data:blog.pageType == "archive"'>
..execute if satisfy the condition
</b:if>

5. Homepage

<b:if cond='data:blog.url == data:blog.homepageUrl'>
..execute if satisfy the condition
</b:if> 

6. Specific page/url

<b:if cond='data:blog.url == "BLOG_PAGE_URL"'>
..execute if satisfy the condition
</b:if>

7. Specific Label Page

<b:if cond='data:blog.url == "http://BLOG_NAME.blogspot.com/search/label/LABEL_NAME"'>
..execute if satisfy the condition
</b:if>

8. First post (checking if the post is the first post)

<b:if cond='data:post.isFirstPost'>
..execute if satisfy the condition
</b:if> 

To specify the condition as not true, change the comparison operator from == (is equal to) to != (is not equal to).

Logical AND Conditional tags

<b:if cond='data:blog.url == "BLOG_PAGE_URL"'>
<b:if cond='data:blog.url == "BLOG_PAGE_URL"'>
..execute if satisfy the condition
</b:if>
</b:if>

Logical ELSE Conditional tags

<b:if cond='data:blog.url == data:blog.homepageUrl'>
..This is homepage
<b:else/>
..This is not homepage
</b:if>

Applying the Conditional tags

Simply put the content/code/HTML between the opening tags and the closing , e.g :

<!-- facebook share -->
<b:if cond='data:blog.pageType == "item"'>
<a expr:share_url='data:post.url' name='fb_share' rel='nofollow' type='box_count'/>
<script src='http://static.ak.fbcdn.net/connect.php/js/FB.Share' type='text/javascript'/>
</b:if>
<!-- /facebook share -->

Wrapping a widget with the conditional tags

Although some widget, e.g: Adsense/Facebook Like etc can be wrapped directly,
this particular widget with a common general structure required a little trick on it :

<b:widget id='something' locked='' title='' type=''>
<b:includable id='main'>
..The widget here
</b:includable>
</b:widget> 

It can be done like this

<b:widget id='something' locked='' title='' type=''>
<b:includable id='main'>
<b:if cond='data:blog.pageType == "item"'>
..The widget here
</b:if>
</b:includable>
</b:widget> 

Credit : This is as a refference only. I've compile this out from these blog..



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