S- Tag and C-Tag are very common terms found in Metro Ethernet equipment. As others have pointed out, it is a slightly different VLAN protocol than 802.1q and is a part of 802.1ad.
S-Tag = Service Provider Tag (or outer tag)
C-Tag = Customer Tag (or inner tag)
This is also commonly found in DSL DSLAMS, MSAPs and GPON deplopyments