Here are some of the more commonly used and sometimes less often explained sewing terms
Seam allowances are the distance between the edge of the fabric and the stitching line. Seam allowances are sometimes also called "margins" (but not very often). Other kinds of "allowances" are the amount of fabric allowed for something, e.g. a hem.
Imagine you were weaving your own fabric. To start you set up the lengthwise threads on the loom. Then you start to weave the sideways thread in and out, between each lengthwise strand. When you get to the end of one row, you have to turn around and go back across. As you keep doing this you build up your cloth.
The lengthwise threads are called the warp, and the "across" threads are called the "weft" (in the old days they were called the "woof"). I remember it by thinking that when you have the fabric hung up on the loom, the warp threads may warp, and when you are weaving, you send the weft threads "weft and wight" (left and right).
The left-hand and right-hand sides of the cloth where you turn the shuttle to go back across are called the selvedges and they are naturally tighter than the rest of the cloth. If they weren't they might be loopy and get caught on things, plus the cloth would be of poor quality. I think sometimes, the selvedges have thicker warp threads for strength.
The grain-line is the lengthwise thread and when you have your pattern lain on the fabric, the long line with the arrows has to match this. You make sure they match by measuring from one end of the line to the fold of your fabric, then repeat with the other end. If they match, your pattern is properly aligned. If the line has the arrows pointing away from the line at a 90 degree angle, the arrows are pointing towards the fold of the fabric, indicating that this piece is to be cut on the fold (and not have the fold cut).
There is also the cross-grain, which is perpendicular to the grain-line. It is the weft threads.
The bias is at a 45 degree angle to the grain-line. It's stretchier than the grain-line and the cross-grain and that is what they use for bias-binding, hence the name "bias-binding".
Selvedge: When I started my first dressmaking project, I didn't know what the selvedge was. It's the side edge of the fabric. If you look closely you can see it has little holes in it (where it was held on tenterhooks when they dyed it). It is best not to use the selvedge as a seam allowance because (I think) it can shrink more than the rest of the fabric so it would ruin your seams. By the way, before you cut your fabric it is usually advisable to wash and dry it so that if it shrinks, your garment will not shrink after you cut the piece out.
MARKS ON THE PATTERNS
A notch is the little triangle on the cutting line of the pattern piece. When you are putting the garment pieces together, you match these up.
Dots or Circles are important too. They help you to get facings and pockets and things in place properly and accurately. You punch a little hole in the circle with your scissors or awl, and mark the fabric underneath with a tailor's tack. Then, when it comes time to put the pocket or whatever it is on your garment, you just match up the tailor's tacks. You may find it helps to use pins to make sure things are lined up.
OTHER SEWING TERMS
Facings are pieces of fabric cut to match or nearly match the shape of the edge of the fabric you are neatening, such as at a neck-hole or an armhole. They are sewn on, clipped or notched, and turned inside the garment. Then you can under-stitch the facing to the seam allowances.
A Hem is a turned up or tuned under edge of fabric. It usually means the turned up, bottom edge of your garment. A false hem is not really a hem; it's a faced edge at the bottom of your garment.
Under-stitching is what you call it when you sew just the facing to the seam allowances. You can do it by machine most of the time, but sometimes, like on collars, you have to do it by hand because you can't get to it by machine. If you don't want to under-stitch, you can top-stitch instead. That just means that you sew so that you can see the stitches on top.
Interfacing is a type of material that looks like tumble dryer sheets and goes between the garment and the facing. It is usually applied to the facing by either pressing (if it is iron-on) or basting (if it is sew-in). It's purpose it to give the garment structure, i.e. keep it in shape. It's very important for waistbands unless you are using Petersham (also called waistband ribbon).
Ease is very slight gathering. It is usually done at the top of a sleeve. It has to be very slight because you are not meant to get any puckering. You can ease and stay-stitch at the same time (called "super stay-stitching") by putting the sleeve cap under the presser foot on its own at the first notch and, while you are sewing a little way inside the seam allowances and nearish the stitching line, push with your finger against the back of the presser foot so that you stop the fabric from going through so much. When it gets too built up, let it go a bit and push again like before for the next lot. Keep going until you get to the other notch. Then your sleeve is ready to go into the sleeve hole (called "setting the sleeve"). Even if you have a sleeve with no ease, it seems you have to still ease it in because of the seam allowances.
Stay-stitching is when you sew along the edge of one layer of fabric to stop it from stretching out of shape. Have this line of stitching near the stitching line and in the seam allowance. You must stay-stitch all curved edges and bias edges. If you are sewing loosely woven fabric, it is best to stay-stitch all edges (especially if you are sewing plaid, as I found out a while ago.)
Basting is temporary sewing. When you have pinned the fabric together, you can sew it by hand with a longer-than-usual running stitch to keep it in place without pricking yourself. If you are basting pleats in place, you can do so more securely if you sew a cross-stitch on the spot a few times. Some people like to baste on their sewing machines by using a longer straight stitch, but I prefer to baste by hand because it is easier and, in my experience, gives better results.
An under-collar differs from a top-collar in that it is a little smaller and is often cut on the bias. When the collar is attached to your garment, the under-collar will be underneath the top-collar. (I mention this because for a while the matter of an under-collar confused me somewhat.) BTW, it is the top-collar that is interfaced to keep it crisp.
Turn of Cloth Imagine you have a sandwich and you fold it in half. The top slice of bread would end up looking shorter than the bottom one at the edges. The difference in visible length in this instance is refered to as "turn of cloth" (when it is fabric). Undercollars have to be a bit smaller than top collars so that when the collar is turned over, it lies properly. If we didn't allow for the turn of cloth, the top collar would pull the seam upwards and the undercollar would be all wibbly-wobbly (for want of a better word) underneath. The usual allowance for turn of cloth is an eighth of an inch (about 3mm), but more may be required for thicker fabrics such as coat fabrics.
Grading a seam is when you trim one of the seam allowances after sewing the seam. This makes the seam lie more smoothly when it is pressed to one side.