In Elrond Network the validators are responsible with maintaining the chains across all shards. In each shard only one validator can propose a block at a time and a subset of all validators from the shard can aggregate their signature on the newly generated block. There are 2 types of validators: (i) shard validators aggregate transactions in blocks and (ii) metachain validators coordinate the cross-shard communication by aggregating shard headers in metablocks. Each validator is identified by a public key derived from a secret key used to participate in block signing.
The network is maintained by shard and metachain validators. Along with the validators, there are special nodes called observers that do not participate onto the consensus (proposing or signing on blocks) but synchronize with the network and additionally can perform some tasks. The observers can send signed transactions to be processed, can monitor the activity of the network and can be used to find other peers on the network and so on. They could also be service providers for applications like wallet or blockchain explorer.