It covers fostered children in long or short-term foster cared but also includes children living with relatives who aren't parents, e.g. children living with a grandparent/auntie/older sibling or whatever.
"A looked after child means a person under the age of 18 who is provided by a local authority, acting in its social services capacity, with accommodation for a continuous period of more than 24 hours, by agreement with the parents or with the child if he is aged 16 or over, or who is the subject of a relevant court order under part IV of the Children Act 1989. Children looked after under an agreed series of short term placements (respite care) are excluded.
Children in care are more likely to be from groups that tend to do less well in education. They are also more likely to have statements of Special Educational Need (27% compared with 3% of all children)"