A Email Send (Mailbox symbol) is a straightforward single send email, that you simply set your smart list, email and schedule, it can also include trigger campaigns (See Release Notes: December 2013 - Marketo Docs - Product Docs), however it shouldn't have batch campaigns or landing pages (as these don't report via the easy dashboard). A Default Program (Briefcase symbol) is for more complex programs that has multiple emails, landing pages and batch campaigns.
The way I think of it is, if I want a quick single email only, I use a Email Send, if it's more than 1 email or needs a landing page, I use the Default program. You can of course use a Default program for single send too but this isn't time efficient.
Where the fun happens is when you use them both, see how I send multi region emails:
If you tokenize your content, each Email Send will inherit the top level program tokens, so I can easily reuse common assets such as banners, but it also allows me to override any of them too with each Email Send. This to me is ultalizing each program type correctly. I hope this helps.