I need to have a page, called "Seminars", that contains only list of seminars and their short descriptions.
Under this page I need to have pages with full description for every seminar.
Should I use a "blog module" to create section like this?
Please suggest!

If the blog module is fine, please explain how to define a separate "BlogEntry.ss" and "BlogHolder.ss" because I also have specific blogs on my website and I changed templates "BlogEntry.ss" and "BlogHolder.ss".

thank you!

I think your best approach, and probably easiest, would be to create the seminar pagetype yourself. Then you can define exactly the information it needs.

But how to create a page, that is going to collect only titles and short descriptions of all the seminar pages?

Create two different Page types: SeminarHolder and SeminarPage.
The SeminarPage is your detail page. Create all the needed fields there.
The SeminarHolder contains several seminar pages and displays its children summarized. Here's how your site-structure would look like:

SeminarHolder
 +-- SeminarPage 1
 +-- SeminarPage 2
 +-- SeminarPage 3
 +-- etc.

To summarize the SeminarPages, you would create a special template for your SeminarHolder (SeminarHolder.ss), with something along these lines:

<% control Children %>
$Title
$Content.Summary
<a href="$Link">Read more</a>
<hr />
<% end_control %>

That would output the Title, summarized Content and a Link to the detail-Page for every SeminarPage in the SeminarHolder. Feel free to extend this with your own fields and markup.

Thanks!