The main API (the so-called REST API) allows access as data to all aspects of the OpenCorporates website (with the exception of being able to submit data). By default it returns data as JSON (but XML is also available). Access to all the data is free for open data projects/work under the same open licence conditions as the rest of OpenCorporates. We also offer competitive payment plans for non-open data projects. The API can be used without API keys but using an API key increases the usage limits for the service.
If you are matching company names to legal entities from an existing dataset, you should investigate the rather excellent Open Refine, which makes this quick, easy and allows data to be filtered and cleansed quicker than any other tool we know of. OpenCorporates provides a highly popular reconciliation API for Open Refine, which allows matching company names to legal entities.