From my understanding of past papers and this years there are two. One is descriptive, describe your classroom, describe a pet etc. The other is instructional, so write instructions to make toast, to walk to school, to make a cup of tea etc.
I think it's marked on grammar, descriptive language, punctuation etc but not sure.
Free familiarisation papers and marking schemes are on the CSSE website:http://csse.org.uk/index.php?option=com_simplefilemanager&view=simplefilemanagers&layout=list&catid=85&showDate=0&showNew=0&showSize=0&orderBy=a.ordering&order=ASC
The actual papers sat for the coming year's entry are usually available to buy from early February.