Digital Piano
Digital Pianos are the cheap way to buy a piano.  There are of course positives and negatives in owning a digital piano.  Before I state the positives and negatives, I will first talk about how a digital piano works.  A digital piano contains ROM (read only memory).  This ROM is permanent and quick memory.  All digital pianos have sampled notes which are WAV files, with 1, 2, 3 or 4 velocity layers, depending on how much the piano costs.  A velocity layer simply means a single note may have 1 to 4 samples mapped to a key on the digital  piano.  This means that if a digital piano has 88 keys, and 4 velocity layers each, there are 352 sampled notes contained in this ROM.  If each sample is about 4 megs each, that is 1.4 gigabytes of memory worth of notes, which is simply impossible for ROM to handle.  Therefore digital piano manufacturers thought of a way to reduce the amount of memory by not sampling every note (therefore just raise the pitch of the next 10 notes until it becomes ridiculously noticeable), reducing the velocity layers, and looping the sample after 1 or 2 seconds.  Yet, given all of this information, digital pianos can still cost quite a bit of money, and even the best digital pianos sound fake to the trained ear.   
Positives Negatives
Digital Pianos are considerably cheaper than regular pianos.  They do not have to be tuned every month or so.  Wear and tear of a digital piano is minimal compared to a regular piano.  There are some other fun and cool options that a digital piano can do such as different instrument patches, connect to a computer through a midi cable in order to compose through a sequencer, and have a sequencer play piano music through the digital piano.  To a true pianist, the digital piano will feel extremely fake, even if it has fully weighted keys.  The digital piano will also sound a bit fake to a trained ear.  The looping sometimes becomes noticeable and annoying especially if you use the pedal a lot.  The sound of a digital piano is not nearly as voluminous as a grand or even upright piano.