Sprue: This is commonly called the parts tree—it is what the individual parts are attached to. It is NOT trash; the excess plastic can be used to make scratch parts, such as antennae, wires, weapons, whatever!
Ejector Pin Mark: To kick the newly formed plastic sprue out of the mold, there are small, flush-with-the-mold pins that pop out. Removing the sprue from the mold is, of course, good. Unfortunately, these little pins are not always as flush with the mold as they should be. The result is you end up with little circular indentations or raised areas on the model. Believe it or not, this is less of a problem on a lower grade kit! On an HG kit, most parts have detail only on one side, which means the mold designer can put the ejector pins on the unmolded side of the sprue. On MG kits, where more parts are intricately detailed and double side (detail on both sides), it is almost impossible to avoid ejector pin marks on a detailed area. A good example of this is the Bandai 1/100 MG RX-79(G) backpack. The backpack is made up of several large, flat walls of plastic, and these need ejector pins to pop out of the mold. The backpack has a plain exterior and a detailed interior. The choice was: Do they ruin the highly visible exterior which is easy to fix, or ruin the interior which is less visible but harder to fix? Bandai unfortunately choose to pepper the intricately detailed interior with ejector pin marks. (fortunate for the snap and sticker crowd, unfortunate for the glue and paint crowd)
Knit line: This is the wavy line seen on the larger pieces of plastic in a kit. It usually occurs near the middle of a large, flat piece and can be seen easily due to the different reflection properties of the plastic on either side of the knit line. A knit line is caused during the molding process. The molten plastic is pumped into the mold plate through numerous ports. When you lay a sprue flat on a table, you'll see little pegs of plastic along the sprue that point out of the plane of the sprue. These are the ports where plastic is injected into the mold plate. As the liquid plastic shoots through the mold plates, it cools and starts to develop a surface texture based upon the direction it is moving. Eventually, the liquid plastic from all injection ports of the mold meet in the middle. Since the plastic has cooled by the time it reaches the center of the mold, a visible boundary called a knit line is formed between the shots of plastic due to their different surface textures. If all the plastic is shot into the mold at extremely high temperature and pressure, then knit lines will not form—as the super hot plastic meets in the middle, it will still be hot and will re-melt itself into a uniform surface. Knit lines generally do not affect the surface of the part other than how light bounces off the part, so they are only an issue if you do not paint your kit. But, of course, you paint your kits…
Draft angle: The molds used in making plastic models are essentially large, flat plates. The details and the sprue are carved into the mold plate in relief. When the two plates are placed together, the hollow space in between is where the hot plastic will be injected to form the parts. Carving straight into the mold produces a part surface that is parallel to the flat plane of the mold. Detail that is molded into a surface parallel to the plane of the mold is going to have crisp, clean detail. You can see this when you lay a sprue flat on a table and look directly (perpendicular to the plane of the sprue) down at it. The detail you are looking at should be clean and crisp. Along the same lines, it is hard to carve detail sideways into a mold plate. Pick the sprue up and look at it along an edge. You'll see that the details start to soften the further away the detailed surface is from parallel to the plane of the sprue. Since you cannot create an undercut on one side of the mold and still be able to remove the plastic sprue, the mold designers can't have crisp, sharp detail beyond a certain angle from perpendicular to the mold plane. The angle away from the mold plane of a part surface is called the draft angle.