Events and Logs
Events are signals dispatched by smart contracts so that dapps, or anything connected to Ethereum JSON-RPC API, can listen for something that has happened on the blockchain, and then act accordingly. These events are then recorded in logs.
Technical Introduction to Events and Logs in Ethereum - Joseph Chow, ConsenSys Media, 2016 [5 min read]
This post from ConsenSys Media gives a great introduction to Events and Logs, and explores three different use cases: return values for the front-end, asynchronous triggers, and cheap storage.
Events - Solidity Documentation [2 min read]
Smart Exchange [Exercise]
This basic contract triggers an event based on which of its functions are called. Make sure you understand what is going on. As a bonus, try to see if you can figure out what the contract does!