In a serverless architecture, the main components are the event source and the Lambda function. The event source can be a custom application or AWS. Each of the AWS event sources uses a specific format for event data. At present, the list of services supported by AWS Lambda as event sources can be found at https://docs.aws.amazon.com/lambda/latest/dg/invoking-lambda-function.html. These supported AWS services are broadly divided into stream-based services and regular AWS services.
Out of all of the supported AWS services, Amazon Kinesis Streams and Amazon DynamoDB streams are the only stream-based AWS services. The rest of the services are regular AWS services.
Apart from the previously mentioned supported AWS services, a custom user application can also create an event to trigger the Lambda function's invocation.