Platform allows you to download and install plugins made for common 3rd-party tools, such as MySQL or Nginx, that will automatically collect and send performance data up to New Relic. That data is then visible in New Relic’s Plugins tab. New Relic Plugins also provide limited alerting.
By comparison, the Agent SDK makes it possible for you to drill down even further and add instrumentation anywhere in your code, so you can see detailed performance data. Your application is then visible in the New Relic APM tab, with access to most of the features in that tab, including Transaction Traces, Custom Metrics, SQL Traces and Alerting.