Spot on Gillianren, SAT tests in England (I think they're more technically called National tests now but most people think of them as SATs as that's what they are more or less they just changed the name) are given to children in years (let me see if I can remember this right, thank goodness for wikipedia)
at age 7, 11 and 14 (year 2, 6 and 9, actually the 3rd 7th and 10th year of schooling due to the presence of "Reception" in the school system for the first year) and basically test whether the childrens education is in line with the national curriculum (which states what children should know at various stages), thats the theory anyway, whether they actually tell anything useful is one of those knotty little problems that has arguements on both sides (including "teaching to the exam" for those against it and the importance of knowing where children are at for those for it) they have no impact on university entrance (that's done on AS/A2 levels and to a much lesser extent GCSEs (basically Universities like to see good GCSEs, especially those that are oversubscribed and may use them as a tie-break, especally those Universities that decide not to look at AS levels.
EDIT due to losing the plot in typing out the ages at which SATs are given