Oven cleaning products tend to have a pH of between 11 and 13. The pH scale runs from 1 to 14, where a pH of between 8 and 14 is considered alkaline and anything below 7 is considered acidic.
Oven cleaners with a pH level of 11 or over are therefore very alkaline, which means that they have tremendous power when cutting through grease and baked-on grime in the oven. You do need to take care when using alkaline cleaning products, as they can be hazardous when breathed in or when in contact with the skin.