Knots are particularly interesting as they mark where branches had formed on the tree your furniture was made from. They're absolutely natural and because every tree has branches you're highly likely to find a knot somewhere on your item.
Tiger marks are essentially stretch marks in the wood and represent rapid growth. This too is natural. Whilst tiger marks and knots might not be present in your furniture, wood grain certainly will. Every item of wood has its own unique wood grain, in the same way that every human has their own unique fingerprint.
Every tree is different, growing and maturing at different rates. This creates grain variation, not only in light and dark shades but in thickness of grain. These variations are not a fault, its nature's way of showing growth patterns (growth rings).
Wood expands and contracts with changes in the surrounding humidity. Humid air will cause wood to expand whilst drier air will cause wood to contract. This movement cannot be stopped although assuming your furniture is placed indoors the effects of this should be barely noticeable.