The Word DMA (WDMA) interface was the fastest method used to transfer data between the computer (through the ATA controller) and an ATA device until UDMA. Single/Multiword DMA took over from PIO as the choice of interface between ATA devices and the computer. The WDMA interface is grouped into different modes.
The explanation for the difference between single and multiword DMA can be found in how the Intel 8237 DMA chip works. In single transfer mode, only one word (16-bit) will be transferred between the device and the computer before returning control to the CPU, and later it will repeat this cycle, allowing the CPU to process data while data is transferred.
In multiword transfer mode (Block mode) once transfer has begun it will continue until all words are transferred.
Two additional Advanced Timing modes have been defined in the CompactFlash specification 2.1. Those are Multiword DMA mode 3 and Multiword DMA mode 4. They are specific to CompactFlash. Multiword DMA is only permitted for CompactFlash devices configured in True IDE mode.