Setting up Teasers is one of the functions in Thesis that I always have trouble remembering - maybe because you need to select options in both Thesis and WordPress to achieve this.
Anyway, you have some flexibility in how you configure your Teasers layout:
You can have a front page which is all Teasers (as on this site), or you can have a front page with one or more posts (in full or an excerpt), followed by a number of Teasers.
If you have posts followed by Teasers, the posts are referred to as Featured Posts.
You also have a number of options for configuring the Teasers themselves - for example fonts, font sizes and the post META to display.
So here's how you set up Teasers in Thesis:
Go to the Home Page Display Options on the Thesis Design Options screen:
Click the '+' sign next to the Features and Teasers heading and select the number of Features you want from the drop down menu (choices are from 0 to 8).
In the Display Options section (immediately above the Home Page Display Options), click the '+' next to 'Posts' and choose either 'Display full post content' or 'Display post excerpts':
Go to your WordPress Settings > Reading screen and choose how many posts to show in the 'Blog pages show at most' field:
And here's the important bit:
If you choose the same number of posts to show in the 'Blog pages to show at most' field as the number you chose in the 'number of featured posts to show' drop down menu on the Design Options screen, then all the posts on your front page will be displaid as featured posts.
If you choose a bigger number of posts to show in the 'Blog pages to show at most' field, than the number you chose in the 'number of featured posts to show' drop down menu, then the difference will be displaid as Teasers.
So, choose 8 posts to show in the 'Blog pages to show at most' field and choose 8 featured posts from the drop down menu, and you'll get 8 featured posts (or excerpts) on the front page.
If you choose 8 posts in the Settings > Reading screen and choose 4 featured posts, you'll get 4 featured posts and 4 Teasers.
I know - not the most intuitive of processes, but there you go.
Finally, you can define the styling for each of your Teasers in the Teasers section of the Design Options screen:
Click the '+' sign against each of the sub sections and select the characteristics you want. Don't forget the Big Ass Save button when you're done.
OK, that's it - let me know if you need any clarifications.