pretty simple really - you've identified the most important feature - the "tare" function.
regardless of what is on the scale (subject to range) push a button and the object on the scale goes to weight: 0
very convenient for adding ingredients without a lot of head math.
three types of scales - balance, spring, digital
balance is like the doctor's office - move (counter)weights until the balance point is reached.
given decent construction, most accurate of the bunch. convenient? no. size, big.
spring scales:
don't go there. accuracy, linearity, repeatability - all major issues.
digital scales:
only kitchen way to fly, imho.
digital designs separate themselves into three major classes:
3000dd, 5000dd, 10000dd
you asked? okay, what that means is for the total _range_ the scale will measure and display 1/3000 or 1/5000 or 1/10000 of the weight.
for usual and customary home bread stuff, you need a scale that will handle up to 11 pounds / 5 kilograms. keep in mind, the "range" of the scale must include the weigh of the bowl + the weight of the stuff inside.
such scales are available in many mass marketers (ie K-Mart) or specialty stores (ie Bed Bath & Beyond) and virtually anywhere on the web.
so what's with the dd nonsense? the XXXXdd spec, sometimes just shown as a singled - XXXXd - is the number of divisions of maximum weight/capacity the scale will display. so a 5 kilogram = 5000 gram = 5000dd / 5000d scale can realistically show a resolution of one gram (5000 total weigh / 5000dd = 1)
forget about "absolute accuracy" - whatever scale you have is likely to be very consistent and whether it shows an absolute known quantity of 4000 grams as 3999 grams or 4001 grams is like . . . way past important.
no name, made in China, $4 models will display one gram but only measure to 3000dd.
legal trade scales typically run in the 10000dd range. if they are "certified" for legal trade, costs more.
nice info, but you're unlikely to see those kind of specifications on a package. check the manufacturer web site and model specifics.
you'll find entirely useful and satisfactory digital kitchen scales from $20 to $50 - the stainless steel body + cute glass platform costs more; the "it's all plastic' runs to the lower end.