An OpenCL miner is a bitcoin miner that uses the OpenCL framework to perform the hashing computations. When used with a modern GPU, this can produce hash rates orders of magnitude higher than what can be achieved with a CPU.
OpenCL is a computing framework which allows programmers to write code which works across a variety of hardware, both CPUs and GPUs included. OpenCL was originally developed by Apple.
The use of an OpenCL miner allows one to hash using a GPU (Graphics Processing Unit), which has a vastly larger ability to perform algorithmic calculations than an CPU (Central Processing Unit).
Puddinpop has also released a C++ OpenCL client capable of being compiled into the GUI client or daemon, or as a stand alone RPC miner.
See the Mining Hardware Comparison page for detailed statistics on the hash rates that can be achieved with various hardware.