The grades of H1 subjects are worth half as much as the grades
of H2 subjects when calculating university admission points.
The content of H1 subjects are half of that of H2 subjects, but
of equal difficulty.
GP and 2nd language are H1 subjects. Science subjects can be H2
or H1.
In JCs (ie. school candidates) you must study a contrasting
subject (eg. econs is a contrasting subject for science students),
which can be either H1 or H2.
Most students choose (or their JCs only offer such subject
combinations) to have their contrasting subject as their H1
subject.
You can download the syllabuses (both H2 and H1) for all the
subjects here :
http://www.seab.gov.sg/aLevel/syllabus/schoolCandidates/2011_GCE_A.html