If you’re using separate templates for the synopsis and article body, you’re work is pretty easy and only slightly tougher if you’re planning to use the same template for both.
In the former, you’d simply have different parameters and variables specified in the weblog:entries tag. So in the listing template wherein you would have the synopsis of several articles listed, for example, chronologically by entry date with the most recent at the top, you would pull your summary and thumbnail into the template. In the “details” template you would pull in the body and full image (not to mention whatever other fields you deem appropriate).
In the latter, you could accomplish what you’re looking to do, at least generally, using conditionals as you have suggested. Only it would be conditions around segments rather than body versus synopsis. When you’re using the same template to serve two purposes, you still have to give EE a way to tell which purpose is active at which time. An easy-ish way to do this is the use segments. So when you’re using your index template in your blog section, for example, the URL would look like this:
http://www.my-domain.com/index.php/blog/
And so in that template, you would have a weblog:entries tag, pulling content from your “blog” weblog.
{exp:weblog:entries weblog="blog" orderby="date" sort="desc" limit="10" disable="trackbacks|member_data|categories"}
{summary}
{/exp:weblog:entries}
</code></pre>
And since the link is not pointing to a new template, but instead pointing at itself, you could add a conditional that says if you are displaying the article, which would be the url title added to the end of the address and therefore occupying, in this example anyway, segment 2, show the full article, and if not, show the abstract - and when displaying the item by url title, it will only show the one article, whereas without that limitation, it would show all articles in that weblog meeting your other parameter values:
{exp:weblog:entries weblog="blog" orderby="date" sort="desc" limit="10" disable="trackbacks|member_data|categories"}
{if segment_2="{url_title}"}
<h2><a href="http://{homepage}/blog/{url_title}">{title}</a></h2>
<p>{site_url}/images/uploads/fullsize_image.jpg</p>
<p>{body}</p>
<p> </p>
<p>{if:else}</p>
<h2><a href="http://{homepage}/blog/{url_title}">{title}</a></h2>
<p>{site_url}/images/uploads/thumb_image.jpg</p>
<p>{summary}</p>
<p>{/if}</p>
<p>{/exp:weblog:entries}
That’s a simplified example, but it should give you the idea. I generally use a separate template for listing view versus individual entry view, if only because i find it easier to organize my thoughts that way and there are other benefits that are a bit useful too. But it can be done in either fashion.
The caveat - I’m only 8 months into my EE life, so there may be better ways to think about this than I’m proposing here - but I’ve made out ok with this approach. I’d definitely recommend going through Michael Boyink’s free tutorials at www.train-ee.com and if you can swing it, his paid screencasts are a bargain too. I learned a ton very quickly from him, and from the community in the forum.
Hope this helps.
Cheers,
John