What is an SPV client? I've seen SPV client mentioned in several places. 
What exactly is a SPV client? How is it different from a standard client?
Simplified Payment Verification:


  A Bitcoin implementation that does not verify everything, but instead
  relies on either connecting to a trusted node, or puts its faith in
  high difficulty as a proxy for proof of validity. BitCoinJ is an
  implementation of this mode.



http://en.bitcoin.it/wiki/Scalability#Simplified_payment_verification


Also read:


https://en.bitnovosti.com/2018/07/06/jonas-schnelli-current-and-future-state-of-wallets/
https://github.com/bisq-network/bisq/issues/1062
http://en.bitcoin.it/wiki/Thin_Client_Security#Simplified_Payment_Verification_.28SPV.29


MultiBit, Bitcoin Wallet for Android, and Electrum are examples of SPV clients.