A shell totally covers something and is quite hard. For example, the outer covering of a nut (peanut, cashew, almond, etc.) is a shell and the nut is completely covered and protected by the shell. A crust is relatively soft and easy to break through, even though it is harder than what it is protecting. Often, a crust covers the top layer of something (think of the crust that forms on a piece of food as it is grilled or pan fried) but does not cover all sides. Oddly, in geology, the top layer, which can be many, many kilometers thick, of the Earth (as well as other, rocky, celestial satellites) is called the crust. It is clearly extremely hard and covers all sides of our planet, yet it is much softer than what lies beneath the crust.

Shell usually relates to something used by animals or humans. Many times it is protective but not always. Examples are: A sea shell, which is something sea creatures live in. A gun shell, which is like a bullet -- something fired out of a weapon. A coat -- such as a windbreaker -- can be called a shell. Crust usually breaks more often than a shell. When I think of crust, I think of the crumbly outer part of a pie. As a rule, crust will usually crumble or break. Examples: Pie crust, as above. The Earth's crust, which is the outer layer of the Earth -- the thing we build on. Similarly to pie crust, it is not made up of one piece, but many smaller ones that fit together. As such it can be broken easily. Crust can refer to build up of hard stuff, like the stuff in your eyes after you sleep.

A shell is something that is intended - a sea shell, egg shell, nut shell, etc. A crust on the only hand is a layer that has formed usually over a liquid - the earths crust sits on top of (liquid) lava, a crust is formed around the glutinous filling of a pie, or a scab is the crust of dried blood on the skin. It is probably easier, therefore, to think of the difference between the two not as a list of items but according to their properties - are they intended (shell) or a consequence of some other action (crust).