The differences include:
1) Podcasts can be shared on iTunesU with a Feed URL.
2) The feed URL or the RSS feed can be used to share the podcasts elsewhere.
3) Podcasts can have advisory labels as well as a category, or iTunes U category associated with them.
4) Podcasts will have a different icon beside them in the frontent Library and the admin panel to distinguish them from collections.
5) How podcasts are created - you can add a podcast in the same way that you add a collection; however, when you are creating the podcast you need to check off the box next to "Make this collection a podcast" and fill in all of the podcast details below.
The similarities between Podcasts and Collections include:
1) How media is added to a Podcast - when you turn a podcast into a collection it will act as a regular collection when it comes to uploading and storing media.
2) How users are added to a Podcast. Users are added to Podcasts in the same way they are added to Collections.