Put simply, the data link layer is the first level of abstraction over the raw wires (or wireless medium) that your data travels across. If we want to think of the simplest model (two machines sharing a wired connection to each other), the physical medium itself, i.e. the raw wire connection (or again, the wireless medium if you prefer that analogy) is the bottom of the stack. The Data-link layer rests on top of that, and that defines how the communication will be transmitted. When you hear terms like "Ethernet" or IEEE802.11(x), those are framing mechanisms that allow data to be formatted and sent across the medium and received by another device.
A much more detailed explanation can be seen at http://en.wikipedia.org/wiki/Data_link_layer
A concept is called OSI to discribe the process of data communication. OSI concept is consist with 7 layers. They are Application layer, Presentation layer, Session layer, Transport layer, Network layer, Data-link layer & Physical layer. Application layer is a Top layer & the physical one is a bottom layer.
Data link layer is a some kind of interlligent layer. It's used to control data flow & error control between two nodes. Data link layer is only to know exchange the some kind of data unit called FRAME. The main function of the data link layer is transfer the frames between two node through a physical layer. Data link layer can be know MAC address.