Relay Or Participant Node

What's the difference, which do I need? 

The Algorand network is comprised of two distinct types of nodes, relays and participant. A participation node is a non-relay node that hosts one or more online accounts with valid participation keys.

Non-Relay/Participant Nodes

For the current TestNet release, non-relay nodes can be configured to be archival or non-archival nodes. Archival nodes store the entire blockchain; non-archival nodes store only as many blocks as necessary to ensure progress. Thus, a non-archival node requires significantly less storage, generally less than a full day’s worth of blocks, and can reasonably bound its database size.

Currently, a relay node must be archival; a non-relay node may be archival or not.

Relay Nodes

A node is a valid relay node if two things are true:

1. The node is configured to accept incoming connections on a publicly-accessible port (4161 by convention).
2. The node's public IP address (or a valid DNS name) and assigned port are registered in Algorand's SRV records for TestNet.

Relay nodes are where other nodes connect. Therefore, a relay node must be able to support a large number of connections and handle the processing load associated with all the data flowing to and from these connections. Thus, relay nodes require significantly more power than non-relay nodes.