What is Multi-Protocol Label Switching (MPLS)?
MPLS is a packet-forwarding technology which uses labels in order to make data forwarding decisions. With MPLS, the Layer 3 header analysis is done just once (when the packet enters the MPLS domain). Label inspection drives subsequent packet forwarding. MPLS provides these beneficial applications:
Virtual Private Networking (VPN)
Traffic Engineering (TE)
Quality of Service (QoS)
Any Transport over MPLS (AToM)

Additionally, it decreases the forwarding overhead on the core routers. MPLS technologies are applicable to any network layer protocol.
What is a label? What is the structure of the label?

A label is a short, four-byte, fixed-length, locally-significant identifier which is used in order to identify a Forwarding Equivalence Class (FEC). The label which is put on a particular packet represents the FEC to which that packet is assigned.