MailChimp is a free service that lets you quickly and easily create email newsletters & marketing campaigns.
In order to send out an email campaign, you need to have a list of people whom you are going to send it to, this is called your Subscriber list.
On any one list, you may have different categories of people with whom you want to send out slightly different emails to, MailChimp call these categories Groups.
People can opt in to be included in one or more Group when they subscribe to your list.
Sometimes you may wish to send an email to a collection of people on a list who might not all be in the same group, but who meet certain criteria which you specify. Just like in Microsoft Excel, Mail chimp lets you apply filters to your mailing list to show only those people who meet your search criteria (if you are familiar with Databases then this is essentially the same thing as writing a query for your Subscriber List). Mailchimp calls the results of these filters Segments.
Once you have identified the list of people who you wish to email then it's time to write the email you are going to send. As Mailchimp assumes you'll be sending more that one email to these people over the course of time, it refers to these emails as Email Campaigns, or Campaigns for short.
You can reuse and remix email Campaigns by saving them as Templates. Templates allow you to send out a series of similarly formatted emails, so you only have to tweak the content and don't have to spend hours formatting and positioning everything.