Here are some things that will be on most of the patterns you make. You need to know why they are there. Hopefully knowing what they are will give you a better handle of whether or not you need them and where to place them.

Seam Allowance; This is something that you need in every pattern you make. It's basically an extra amount of fabric that will go into the seam. 

Darts; When seen on paper it's basically a triangle. The purpose of it is for it to wrap around the body better. One popular place to put it is at the bust point. This tumblr post right here explains it really well. This is the image they used to describe it.

  Notches; Marks that indicate where two separate pieces should meet. It will just help you line up the fabric on a curved piece, because the curves have to match up more than the ends. A popular place to use it is on the sleeves.

Fold line; It'll tell you when something goes on the fold it will also help you position it better on the grain of the fabric

Grain Line; It's like the fold line only that it will tell you in relation to the grain of the fabric.