Oil, Varnish and Shellac
Most furniture finishes fall into three broad families, and choosing between them is easier once you stop comparing brand claims and start comparing what each one physically does to the surface.
A drying oil soaks into the timber and hardens within it rather than on top of it. Linseed and tung are the traditional examples. There is essentially no film, so the surface keeps the texture of the wood, and light passing into the timber comes back out having travelled through the fibres, which is what people mean when they say an oiled surface looks deep. The cost of this is protection: with no film there is nothing between a wet glass and the wood, and rings appear readily. The benefit is repair. A worn patch can be cleaned and re-oiled without stripping anything, and the repair is invisible.
A varnish is the opposite proposition. It cures as a continuous film sitting on the surface, and that film is what resists water, heat and abrasion. This makes it the sensible choice for a dining table or a kitchen surface. It is also what makes a varnished surface look, at close range, as though the wood is under something rather than finished. When a varnish film is damaged, the repair is a repair to the film, and blending a patch into a cured film is genuinely difficult.
Shellac sits between them and behaves unlike either. It is a resin dissolved in alcohol, so it dries by the alcohol leaving rather than by a chemical cure. Every new coat partially dissolves the one beneath and merges with it, which means shellac has no layer boundaries and can be repaired by simply applying more. It gives a warmth that neither of the other two matches. It is also dissolved by alcohol in service, which rules it out anywhere spirits are likely to be set down, and it softens under a hot dish.
There is a fourth thing sold in large quantities that is none of these: a thinned mixture of oil and varnish, sold under many names. It is applied like oil and builds a thin film like varnish, and it is genuinely useful, but it does not have the depth of a true oil finish or the durability of a true varnish. It is a convenience finish, and it is worth knowing that is what you are buying.
The useful question is not which finish is best but what the surface has to survive and who will repair it. A bookcase and a dining table have almost nothing in common as finishing problems, and a finish chosen for one is usually the wrong answer for the other.