The recent advancement in GPU technology has attracted researchers who need intensive computing to the GPU-based computing (GPGPU) field because of its high and inexpensive performance. However, GPGPU programming platforms are traditionally vendor- or hardware-specific, which complicate the access to the computer power of heterogeneous processors from a single host. The recently released OpenCL is expected to become a standard for massively parallel heterogeneous processors. This tutorial introduces the OpenCL, explaining the characteristics of the environment and describing in detail the basic structure of OpenCL program. The tutorial also presents and evaluates various techniques to improve the performance of OpenCL applications.