A connector is software that provides a connection between a Mule flow and an external resource. The resource can be any source of content, such a database, protocol, or API.

Connectors operate within Mule applications, which are built up from Mule Flows, and external resources, which are the targeted resources.

Basically A connector has two sides. The Mule-facing side communicates with a resource’s target-facing client side to enable content to travel between the Mule flow and the resource.

Anypoint Connectors are extensions of Mule that let you work with external systems such as Salesforce, ServiceNow, Workday, Twitter, and more to send and receive information from these systems to create something new.

Anypoint Connectors let you connect Mule applications to APIs and resources. You can use connectors within your Mule applications to send and receive data over a protocol, or to and from an API.

Anypoint Connector DevKit provides the tools and interfaces for building custom connectors.

As reusable components that hide API complexity from the integration developer, custom connectors facilitate integration with SaaS and on-premise Web services, applications, and data sources. Connectors that you build using Anypoint Studio and in Mule ESB runtime environments, function as extensions of the core product.

There are numbers of Mule 3 connectors available in Mulesoft Exchange. You can validate by looking this link

Here is sample screenshoot for connectors available in Mulesoft

If you got some knowledge about Connectors, please follow with me in next topic to understand how a Custom Connector created in Mule3.