Is the purpose of this load balancing or redundancy?
Actually it can be both. It depends on how spanning tree is set up between the switches. Cisco supports per-vlan spanning-tree which allows forwarding on trunk ports per vlan. If you have 4 vlans, 2 can go through 1 trunk and 2 can go through the other. For example:
Vlan 1 & 2 = forward trunk 1
Vlan 3 & 4 = block trunk 1
Vlan 1 & 2 = block trunk 1
Vlan 3 & 4 = forward trunk 1
The above will get your load balancing and redundancy. Redundancy comes in when one of the switches dies in some way. (port goes out, power failure, etc.) Say switch 2 died a horrible death. Now, Vlan 3 & 4 will go into forwarding.
Also, how is this implemented?
That's a whole other topic, but overall you don't need a routing protocol because this is L2 functionality. Routing protocols will get you L3 redundancy. For example, you can advertise 2 routes to 192.168.1.0/24 from 2 different sources. Administrative distances, metrics, and maximum paths come into play where you can have load balancing if the route advertisements are exactly the same and will also serve as redundancy, or you can have just redundancy and only put one route in the table. All routing protocols have a database outside of the routing table and you can configure each one to only put 1 route in the table when they know of multiple routes to the same destination. There are some protocols, BGP, that does this by default and you have to specifically configure it to put multiple routes in the routing table. So, that's a huge topic
Overall, you can do what you ask with spanning-tree alone and it's enabled by default on all switches. It's the configuration of the switches that will get you the above.
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