1. The selvedge is the edge of the fabric that stops it unravelling. The grain is the way fabric is knitted or woven together. The bias is when the fabric thread line or warp and weft are at 45 degrees to the main seam line.
2. Unless stated otherwise, sewing patterns include a seam allowance and they usually state what it is.
3. Interfacing comes in different weights or thickness: light, medium and heavy weight.