The OpenAI GPT-3 API can be applied to virtually any task that involves understanding or generating natural language or code. It provides a general-purpose “text in, text out” interface, allowing users to try it on virtually any English language task.
You can use GPT-3 API for the following use cases:

Semantic search
Sentiment analysis
Content generation
Translation
Summarization
Classification

The API is powered by a family of models with different capabilities and price points. The base GPT-3 models are called Davinci, Curie, Babbage and Ada. The Codex series is a descendant of GPT-3 that’s been trained on both natural language and code.
Source: https://openai.com/blog/openai-api/