1. Stiff - Stiff blades are the ones which do not offer much flex. A good example would be 7-ply all wood blades which are stiff. These blades are thicker and heavier in comparison to 5 ply wood blades. Adding carbon layers to the blade (carbon-wood combination) also makes the blade stiff. Such blades offer more 'backbone' and are very good for blocking. If you are using such a blade then looping underspin would require precise technique and timing. However, counter driving and counter looping would be easier.
2. Hard - A blade can be termed as hard if it offers solid rebound, a distinctive 'ping' sound and has a strong catapult effect. Hardness can be introduced for example, by adding walnut topsheet to the blade construction. Classical examples for such blades are Yasaka Ma Lin Extra offensive, Donic Ovtcharov Senso V1/V2 etc. Such blades might offer some flex and are very good in generating speed, if your timing is correct. Another comparison can be between blades having Koto and Limba wood as topsheet in blade construction. Koto is distinctively hard than limba wood and thus offers a sharper feeling on ball contact. Limba wood on the other hand offers more soft feeling and it appears that the ball stays marginally longer on limba wood blade thus giving more help in topspining.
3. Flex - This property can also be termed as rebound of the blade surface when it hits the ball. A flex blade would bend more (however it would not be visible to naked eyes) and would offer more zip and spin in the topspins. For example Stiga Allround Classic, Donic Appelgren Allplay, Donic Waldner Allplay etc. Although flex blades shine in spin generation but they require more angle adjustment while blocking.