When to use Helpers instead of Partials

Question

In a rails application, in which situation would you use a partial and when would you use a helper? I find both very similar, since they represent markup fragments.

Answer 1

The other answers represent the conceptual consensus regarding the use of helpers vs. partials. Consider the following for further reading:

Basecamp dev making the case for minimizing HTML in helpers http://37signals.com/svn/posts/1108-what-belongs-in-a-helper-method

Viget dev benchmarked each and found that helpers are faster than partials http://www.viget.com/extend/helpers-vs-partials-a-performance-question/