Splunk Apps are bundles of configurations, dashboards, and saved searches that provide pre-built solutions for specific use cases. Think of them as installable packages.
Add-ons (TAs) are simpler packages focused on data collection and parsing. They typically contain inputs.conf and props.conf configurations but no UI dashboards.
You can find both on Splunkbase, Splunk's app marketplace. Common examples include the Splunk App for Enterprise Security and the Splunk Add-on for Windows.