Does wordpress use teaser out of the box or does it require a plugin? Does it always show full posts on front page?
It depends on the theme you are using. But you can get a content limiter plugin. You can also show a static front page (page that never changes) on wordpress.
All you have to do is insert the <!--more--> tag and anything that appears before the tag will appear on the frontpage and then there's a click here to read more link that will take the reader to the full blog post. ~ Bina.