Machine learning APIs are essential tools that provide developers with functions and procedures to leverage statistical techniques for building applications with learning capabilities, without explicit programming. The text highlights several notable machine learning APIs, including Amazon Machine Learning, Aylien Text Analysis, Microsoft Content Moderator, IBM Watson Speech to Text, Kairos, MonkeyLearn, ApiAI, WitAI, Alchemy Text, and TrustServista, each offering unique services such as natural language processing, image recognition, text classification, and sentiment analysis. These APIs vary in pricing and accessibility, with some offering free tiers or open-source options, allowing developers to choose based on their project needs and budget constraints.