Lets go over some general characteristics
Selective barrier
So, to think about this conceptually, I like to think about holding water in my own hands. When you cup your hands together, they do not close all the way, meaning there are small gaps where water can pass through. This is what happens with membranes! When there is a small enough substance (and in the membranes case, nonpolar) it is easy for it to pass through these small holes. Further, if I were to place substances that maybe took up more space than water, or were thicker than water (like sand, rocks, etc), they may have a harder time getting through my hands. This is also the same for membranes! Bigger, more polar molecules have a more difficult time passing through the barrier, which makes it selective.
Flexible structure
We want membranes to be malleable to a certain extent. Our cells bump around, reshape, and move a lot, etc so it is important they are able to “go with the flow” so they do not get easily damaged.
Self-assembling
Lipids, as we learned from the last section, really do not love being around water. Due to this, when in aqueous solution (our bodies!) membranes will assemble quickly to “protect” the fatty acid legs of the membrane and keep the phosphate heads out to interact with the water.
This video shows a depiction of self assembly to avoid the fatty acid tail (red) from interacting with water.
Asymmetric
This may seem counterintuitive as we think of the lipid bilayer as being two layers of identical molecules that come together. However, the inner and outer sides have different compositions due to the differences in what they are interacting with.
Dynamic
This just tells us that proteins etc found in the membrane are able to move around, and are not stuck in one specific location.