Unlike a full node, a lightweight node does not store the entire blockchain and therefore cannot fully verify any transaction. Two types of lightweight nodes exist: those fully trusting an external service to determine wallet balance and validity of transactions (eg blockchain information) and the apps implementing Simplified Payment Verification (SPV). SPV clients do not need to trust any particular service, but are more vulnerable to a 51% attack than full nodes.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc
A client program that allows users in low-capacity environments to still be able to execute and check the execution of transactions without needing to run a full Ethereum node (Geth).