There are lots of services on the net that allow you to integrate your own web site with theirs by sharing data or performing tasks automatically. This concept is called an Application Programming Interface (aka API) and can be a really useful concept.
We have worked with many API's, Twitter and MailChimp are the two we've worked with most and have become pretty good at. Twitter holds the thoughts and opinions of millions of people, their API is a great way to harness that for your own web site or product. MailChimp is the best solution to email newsletters and campaigns that we have ever seen. We're not into re-inventing things so for anyone who is looking to communicate with their customers via email we always recommend this service.
It's not always about making use of other web site services, sometimes it's useful for your own web site to provide an API so that others can make use of your service. This might be something internal to your business, it may be open to the public for free or there may be a pricing modal for it. We've built bespoke API's for many different web sites and services.