I have googled this and although the terms are widely used I cannot find a definition for them. Does anyone have a definition for these terms?
not really sure about it, but in my opinion the difference is small:
In a PoA System you got Sealers and Signer - Nodes. Sealer are predefined in the genesis Block. So A Sealer Node is without a vote of the network allowed to mine/generate new blocks. If you want after a couple of time add new "Sealer"-Nodes, you need to add signers-Node.
A Signers Node is only allowed after n+1 (51%) Sealer-Nodes accept it. If in a Network are more signers, also the votes of the signer count.
So a Signer Node is practical the same like a sealer node, just with the different, that sealer node are defined in the genesis block and per-se allowed to mine/signing new Blocks and signers node first need a postive voting to mine new blocks.
BTW: only signers node can also be disabled to mine new block by a voting.
hope could help.
Addendum: PoA Behaviour is described in the following github issue