The feel of the keys is one of the primary distinctions between pianos and electronic keyboards. The mechanical components of piano keys, such as the wooden hammers that strike the strings within an acoustic piano, cause resistance. If you're coming from a regular acoustic piano, the degree to which a digital piano resembles the motion of a typical acoustic piano makes a major difference in its playability.
Weighted and semi-weighted motions give a more typical piano-like response. Hammer action enhances this responsiveness even more by including genuine mechanical hammers. When played, a little hammer implanted in the key produces a faint "thunk," replicating the reaction of an acoustic piano's keybed.
The best keybeds are commonly referred to be "fully weighted." They will also have graded weighting, such that lower octave keys are heavier, mimicking the heavier hammers and strings found in acoustic pianos. This is commonly known as "graded hammer action."
Touch or velocity sensitivity is another key consideration. The greatest digital pianos can detect minor changes in velocity that affect the attack and loudness of each note. Volume level controls on low-cost digital pianos are a dead giveaway that they lack true velocity sensitivity. (Higher-quality digital pianos also feature volume controls in the form of a knob or slider that regulates the piano's total output.) However, in models with velocity sensitivity, momentary volume is dictated by how hard you press each key.