Before I get into explaining what glulam beans are, it's important that I first explain a little about building in general and most particularly about the framing process because this is where this type of engineered building material is used. You see lumber just isn't what it used to be.
The first thing you need to know about framing, is that whatever it is in a design concept that you have, it must first be approved, and signed off on by a structural engineer. You see, there are set parameters that you have to work in as it pertains to spans, and loads. Spans are how far your lumber can extend, and loads are how much weight it can hold.
If this all sounds easy, it's not because in a home you really are working with confined areas in a real sense. As an example if you're building a two story home, one area you will find yourself limited in, is the floor space between the first and second floors. Also if you plan on having a cantilevered back deck with no posts under it, you will also be limited as to how far out your floor joists can extend.
Glulam beams simply allow you do more in a given area. That is that they allow you to span further in that confined floor space to go over larger rooms, and they will also allow you to cantilever out further on that back deck that you plan to have outside your master bedroom. They do this for two reasons. That is that they're stronger and they don't flex.
You see glulams are made the same way that plywood is. That is that logs are shaved down into thin sheets at the mill then the sheets are layered together with glue and pressed into sheets. Now 1 ¼ inch plywood is about as thick as it gets. However; when glulam beams are made, the layers are usually pressed 2, 4, or 6 inches thick.
So glulam beams are essentially long thick pieces of plywood, and the important thing, is that they have no grain because of this. You see, it's the grain in wood that allows it to flex. Also if a standard lumber beam was to ever crack, or split, and they sometimes do, they will split along a section of grain.
Glulams aren't the only form of engineered lumber that is made though because there are other types that are used in a similar fashion for further spans, and extra loads. These other types of engineered beams come with names like LVL, and I-Joist but they all have one thing in common.
That is that manufacturers have figured out ways to basically take apart a log and then rearrange it back together again in such a fashion that the maximum strength is gotten from the fibres in the wood. They basically remove the flaws that nature put into the wood as it was growing. You see, trees need the fibres arranged one way for strength, but builders need them arranged in a different way for structural integrity.