Wolfram|Alpha is a "computational knowledge engine" whose long-term goal is "to make all systematic knowledge immediately computable and accessible to everyone. We aim to collect and curate all objective data; implement every known model, method, and algorithm; and make it possible to compute whatever can be computed about anything." It contains 10+ trillion of pieces of data, 50,000+ types of algorithms and models, and linguistic capabilities for 1000+ domains.
The API provides two general classes of queries. At the highest level, you can submit free-form queries like users might enter at the Wolfram|Alpha site itself, and get back full Wolfram|Alpha output in a variety of formats. The second type of query is a lower-level request for a single well-defined result, or range of results, from their entity/property-based data API, such as a caloric value for a food item, or a tide table for a requested location