What are the differences between Merge Join and Lookup transformations in SSIS?

Hi I'm new to SSIS packages and writing a package and reading up about them at the same time.

I need to convert a DTS into a SSIS package and I need to perform a join on two sources from different databases and was wondering what was the better apporach, to use a lookup or a merge join?

On the surface they seem very similar. The 'Merge Join' requires that the data be sorted before hand whereas the 'Lookup' doesn't require this. Any advice would be very helpful. Thank you.