Hi Cleiton,
While BAPIs there for ages, SAP started delivering SOAP based WebServices known as Enterprise Services since 2006-07 time frame. There are thousands of such enterprise services that SAP made available for consuming from various applications. SAP also provided a very good documentation available in a dedicated free solution known as ESWorkplace. However, very recently SAP retired that tool and instead provided a kind of alternative at ES Bundles Available Today - Enterprise Services WIKI - SCN Wiki.
There is another kind of APIs that SAP started provisioning known as OData services. These are REST based lightweight web services ideal for consuming from various mobile, rich-internet and other type of applications. These OData services are delivered by SAP mainly as part of their Fiori and other modern applications released in the last couple of years from various business suite of applications and HANA based applications. Details of these services can be found at the Catalog Service - SAP NetWeaver Gateway - SAP Library
Hope this helps.
Nilay