Zapier is a web automation app. With Zapier you can build Zaps which can automate parts of your business or life.
A Zap is a blueprint for a task you want to do over and over. In words, a Zap looks like this: "When I get a new thing in A, do this other thing in B." The first part is the Trigger and the second part is the Action.
Zapier supports hundreds of apps. You can mix and match triggers and actions to automate just about anything.
An example might be "When I get a new entry from a Wufoo form, create a new lead in Salesforce."
Zaps are very lightweight and very easy to setup.
With Zapier, you get as much granularity as you need. You can pick what fields from the trigger service should go to the action service and you can use static text and custom fields too. For instance, you might say that the phone number from your Wufoo form should be the work phone of your new Salesforce lead.
Every few minutes, Zapier regularly checks your trigger for new data (some even get triggered immediately!). When the Zap triggers, Zapier automatically performs the action for you.
Continuing with the Wufoo scenario, say you receive five new entries. Zapier takes each one and makes a new lead in Salesforce, customized to the way you specified in your Zap.
Without any more effort from you, data flows from one service to the other!