Hello Abhijeet,
A PW is a connection between 2 provider edge (PE) devices which connects 2 attachment circuits.
We can emulate below services on PW
1. Ethernet
2. 802.1Q
3. ATM VC
4. HDLC
5. PPP
6. FR PVC
Are they always specific to L2vpn only?
Yes since Layer 2 Virtual Private Network (L2VPN) Interworking allows you to connect disparate attachment circuits.
How do they relate with MPLS?
You woulf require a MPLS enabled backbone for L2VPN to work. Since PW would form targetted LDP session between 2 PE(s).
You may want to have a look at below document which explains operation and overview of L2VPN.
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_l2_vpns/configuration/15-s/mp-l2-vpns-15-s-book/mp-l2vpn-intrntwkg.html
HTH,
