Rendering is used for the smoothest playback of the Timeline. When one adds a Title, whether overlay, or as a stand-alone Clip, and Transitions, a red line will appear above those. This means that for the smoothest possible playback, those elements will need to be Rendered. Basically, what this does is creates a "proxy" file, just for the playback, to simulate how it will look upon Export/Share. This is accomplished by hitting the Enter key, or Return key on the Mac. One can also limit the Rendering to just a portion of the Timeline by setting the WAB (Work Area Bar). This will show the WAB in PrE 4.0, but it works the same way in PrE 9:
As for the screen-cap, here is the URL to that article. Just Copy and Paste that into your browser's URL box: http://forums.adobe.com/thread/592070?tstart=60
If the active link is not working for you, it is likely to be an issue with your browser, or security settings.
By showing us exactly what you are doing with the Titles and Transitions, someone can likely direct you as to how to get the results that you want.
If you're interested, my books explain the various transitions, what they are and how to use them.
I've also got some free Basic Training tutorials that may help get you started.
Jerkiness in a transition can be, as Bill says, an indication that you need to render your timeline.
They can also be related to your source footage.
What kind of camcorder is your video coming from and how did you get it into your computer? Which project setting did you select when you started your Premiere Elements project?
In order to get the best peformance from Premiere Elements, it's very important that your project settings match your camcorder's video specs. Otherwise you can have issues with performance, interlacing and other buggy behavior.