With few exceptions, wood is classified as a hardwood (deciduous trees) or a softwood (coniferous trees).  Scientifically, it's all in the pores and how water is transported from trunk to leaves.  Softwoods have no pores, and water moves by way of fibrous-like cells called Xylem.  Hardwoods have either open or closed pores. The size, number and distribution of the pores are determining factors in the appearance and hardness of a particular wood.  

In further comparisons, hardwoods generally lack resin canals and have greater variation in the size of rays; even to the point of visibility with the unaided eye.   It is believed that hardwoods evolved much later than softwoods and have a greater complexity of cells.

Wood comes from living trees.  That is the most important fact to remember when attempting to understand the nature of your wood boards.  Generally whatever qualities wood possesses, good or bad, they are traceable to the mother tree. Thus, understanding the basic nature of tree growth helps determine the success rate of the project.  Once correctly worked and created into the final object, the initial investment will multiply in pleasure and value many times over.  This is why it is critical to understand wood movement.

Wood absorbs water from the air during high humidity causing it to expand; during low humidity, water evaporates causing contractions.  This process is called "movement in service".  The process of reaching equilibrium moisture content (EMC) is every woodworker's fantasy.  Bringing the wood in to balance with the surrounding air (not gaining or losing moisture) is a slow process with lots of variables.  All work environments are different.

Quarter sawn lumber is more stable than plain sawn or rift cut. Wood that is kiln-dried between 6-12% moisture content is considered optimal.  Incorrect kiln drying can cause a variety of defects such as honeycomb checks, end splits and twists.  A moisture meter is a good investment that could save you much in the way of time and money. 

If your project requires multiple species, remember that each species will have different movement characteristics.  Keep like with like for optimal results, otherwise stress and tension could compromise glue joints or cause cracks with weaker woods.   A larger project requiring multiples planks would lend itself to a careful selection process from the same bundle, same business.  As even within species, pieces of wood will vary in density from different geographical areas.  Allow enough time for the wood to acclimatize to its environment.

The greatest amount of movement occurs across the grain (width).  Thickness has a lot less movement and even less occurs lengthwise.  The narrower the width and thickness, the less movement there will be.  Coating the wood will not stop the drying process, but it will slow it down. 

It is suggested that there are five fundamental issues involved with dimensional change in wood:  preshrinking by drying (kiln or air), control of humidity, mechanical manipulation, chemical stabilization and design.  All will play a roll, whether alone or in combination, in dealing with you're your wood.