SMARTnet is a contract between Cisco and the end customer. It's HW replacement response times goes from 8y5xNBD to onsite 24x7x2hr. You also get:
- Access to TAC
- Software updates
- Smart, proactive diagnostics and immediate alerts for devices enabled with the Smart Call Home feature
Shared Support is a contract between a Cisco partner and the end customer. It's a support provided by the Cisco partner to the end user, with having Cisco in the background.
As far as I know, there are also global support contracts for the largest partners and customers, but these are unique contracts negotiated one by one.