What exactly does professional range mean?
We can't say exactly because it almost certainly depends on exactly which manufacturer or brand-owner is involved. It is also worth remembering that these terms are invented in the first place by marketing departments not by engineering departments and so a lot of the differences are in non-technical areas. For example, how they are advertised, how they are decorated externally, what claims are promoted in the marketing, how support is delivered to the end-user and so on.
Tools that are taken out a few times a year and used for an hour or so at a slow pace don't have the same requirements as tools that are used eight hours a day five days a week for years.
That means that tools labelled professional are expected to be more durable for intensive continuous use. They probably wear out quicker in the hands of a professional worker simply because they are used more heavily and treated more roughly and in more challenging conditions.
Bosch are an interesting case as they often sell very similar tools in both their home-owner (green) and professional (blue) ranges.
Their professional range is typically guaranteed three years. The consumer stuff has (in the EU) only the one year warranty against manufacturing defects.
I guess that some of the differences are likely to be in areas that mostly affect durability more than performance, such as choices of materials (e.g. how much glass reinforcement/fill is in the plastic); size, type and choice of bearings (shielded, sealed etc); bushings vs bearings; quality of switches; potting or laquering of PCBs, etc.
18V Li-ion 18V Li-ion
45 / 19 Nm 46 / 25 Nm
0 – 500 / 0 – 1450 rpm 0 – 400 / 0 - 1340 rpm
10 mm in steel 10 mm in steel
29 mm in wood 30 mm in wood
At the bottom end of the market is stuff that is very poorly manufactured, with parts that don't fit as they were designed to, poorly assembled and not quality-controlled in any meaningful sense. You're lucky if it works out of the box and doesn't fall to pieces the second time you use it. Store-brand tools are usually from a mixture of low-bidding untraceable manufacturers in low-wage countries and are often copies of copies of generic designs that are poorly understood by the maker. There's no incentive for the manufacturer to make anything that lasts beyond delivery to the end user.