Cement/concrete: Cement only refers to the powder that hardens when you add water. If you add sand and aggregate to the mixture, though, you get concrete. So strictly speaking, a cement mixer should be called a concrete mixer.
Sash/window: The part of a window that moves is called the sash. The whole shebang--sash, jambs, sill and everything else--is called a window.
Mullion/muntin: A mullions is a heavy vertical or horizontal member between adjoining window units. Muntins are the narrow strips of wood that divide the individual panes of glass in traditional sash. In the case of so-called “simulated divided lites”, grilles resembling muntins are either sandwiched between double glass panes or else installed over the outer surface of the glass to give a divided look.
Trim/casing: On the outside of a house, the decorative frame around a door or window is called trim, while on the inside, the same thing is called casing. Go figure.
Sliding door/pocket door/bypassing door: The term sliding door refers only to the sliding glass variety that usually leads outside. Those interior doors that disappear into a slot in the wall, on the other hand, are properly called pocket doors. To make things more confusing, the type of paired closet doors that slide past each other aren’t called sliding doors either--they’re called bypassing doors.
Girder/header/beam. In wood frame construction, a heavy horizontal member is called a girder if it’s below floor level, a header if it’s over a door or window, and a beam if it’s pretty much anywhere else.
Wall/partition: Structurally speaking, a wall is always bearing, while a partition is always nonbearing. In most houses, the exterior walls and at least one wall running down the middle of the house are bearing, while all the other walls--er, partitions--are nonbearing. Since these two varieties aren’t always easy to tell apart, it’s prudent to call in an architect or engineer before you go tearing out either one.
Shingle/shake: Wood shingles are sawn by machine and are relatively thin. Wood shakes are larger and thicker than shingles, and are split from a solid block of wood rather than sawn.
Flue/vent: Both of these things stick out of your roof, but a flue exhausts combustion gas from a fireplace, water heater or furnace--anything with a flame--while a vent leads those nasty gases in your plumbing system to the atmosphere.
Banister/Baluster. Banister refers to the entire railing on a staircase. Balusters are the individual uprights in any railing, whether on a stair, a balcony, or whatever. So it’s fine to slide down the banister, but you probably wouldn’t want to slide down the balusters.