Your glass is held in place on one side by the back of a rebate, on the other side by a glazing bead that clips into place (or something like that; this is based on my experience as a shopfront joiner in an aluminium joinery).

The beads can be different shapes, moulded, chamfered, etc. If your door or window has petits bois glued to the glass, where their ends meet the glazing beads they will have to be shaped to match the profile of the beading.

--------------------------------------------------
Note added at 3 hrs (2010-11-28 18:09:15 GMT)
--------------------------------------------------

... of the newly-weds who couldn't tell the difference between putty and Vaseline. When the sun shone, all their window glass fell out.
I read this when I was about 10 and didn't understand the full implications. I'm not altogether sure I do now ...

--------------------------------------------------
Note added at 3 hrs (2010-11-28 18:17:28 GMT)
--------------------------------------------------

Pareclose
BAGUETTE en bois ou profilé métallique ou PVC assurant le MAINTIEN DU VITRAGE DANS LA FENÊTRE.
www.huisclos.fr/spip.php?page=glossaire

Parclose, n.f. : BAGUETTE de bois ou profilé métallique ou plastique qui MAINTIENT UN VITRAGE non mastiqué dans la feuillure de son châssis
http://www.batirenover.com/wps/portal/Batirenover/fiches-sol...

Pareclose
Petite BAGUETTE en bois, profilé métallique ou en PVC, assurant le MAINTIEN DU VITRAGE DANS LE CHÂSSIS
Petit-bois
Eléments décoratifs horizontaux ou verticaux divisant les vitrages (croisillons).
Croisillon
Montant et traverse d’une fenêtre (synonyme de petit-bois). Terme plus particulièrement utilisé lorsqu'il est incorporé au vitrage.
http://www.tryba.com/dn_Lelexiquedelafenetre/#