I think it is to do with the flowers Lin. There are enough keen growers on here give you a better answer. Google heucheraholics.co.uk

All belong to the Saxifragaceae family, but are distinct species. Heucherella is a hybrid between Tiarella and Heuchera - Tiarella does much better in shade, and is usually grown for its variegated leaves and gets taller than Heuchera - it also has no purple or bright yellow colour ranges for its leaves, unlike Heuchera. Heucheralla tolerates full shade, and is grown as ground cover and for its sprays of pink flowers. Heuchera is grown more for its leaf colours than anything else, and most prefer part shade.

Thanks for the info Pimpernel...... :>))

Thankyou Bamboo. Your information was very helpful, at least I will know in future what others are talking about. >:)))