Topics

Topics are indexed parameters to an event. topic[0] always refers to the hash of the hash of the event itself, and can have up to 3 indexed arguments, which will be reflected in the topics.

Logs

EVM uses low-level primitives called logs to map them to high-level solidity construct called Event. Logs may contain different topics that are indexed arguments.

Example:

Consider Event:

 event GroupCreated(uint indexed groupId, uint itemCount);

and you emit the above event inside createGroup function:

 function createGroup() {
        emit GroupCreated(12, 23);
  }

This will create a low-level EVM log entry with topics

0x123....abc (Keccak-256 hash of GroupCreated(uint256,uint256))

0x4238....gef (Keccak-256 hash of groupId), value 12

You'll notice that itemCount will not be a topic, but it will be included in the data section of the event.

Now, in the web3 client you may watch for creation events of groups with groupId 12 Or you can also filter all past events in similar way (refer below):

const event = myContract.GroupCreated({ groupId: 12 });
event.watch(function (err, result) {
  if (err) {
    console.log(err);
    return;
  }
  console.log('Found ', result);
});