A relay node is used for extending coverage but pico also can be used for that. Pico is used for enhancing the network capacity but a relay node can also be used for that.
PICO Cell needs integration with network with adequate configurations like Frequency, Neighbors..etc etc. Whereas RELAY Node acts like a power/coverage extender.
pico cell is an small coverage area which extends from 10-100mtrs and its is an portion of cluster or cellular system... main difference between pico and relay node is whenever service provider want to expand the capacity of a cell they can expand by cell sectoring, splitig, and overlaiding internal to the cellular system but relay node is the external to the cellular system configured to extend the capacity of the cellular system.