The card essentially cuts out the middleman, allowing customers to use their Square Cash at brick-and-mortar stores instead of having to transfer funds to their bank account or rely on Apple Pay. In that sense, it's probably more accurate to describe it as a prepaid card than a traditional debit card.
The sign-up process is relatively straightforward, but does boast a playful element that lets you create your own, custom laser-etched signature for the card, which has a minimal design (no lengthy card number here, just the solitary Visa Debit logo and the chip). Whereas Square CEO Jack Dorsey simply used his @Jack Twitter handle as his signature on the card he teased back in May (he's also the CEO of Twitter, you see), other customers have been a bit more creative with their designs, as you can see in the tweets below. A word of caution: Square screens the signatures, so it's probably best to keep things clean.
Knowing that Square previously abandoned plans for a credit card program, it does come as a bit of a surprise that its Cash Card turned out to be more than just an experiment -- maybe it just wanted to beat its competitor Venmo to the punch.