While a Casino Bonus cannot be withdrawn, it can be converted to cash when you complete certain requirements. Any Casino Bonus with a Wager Requirement (e.g. 10xWR) is bound to expire, depending on its wagering period (e.g. Complete 10xWR within 7 days).
Welcome bonuses are a lump sum of points that you earn by spending money on your credit card. The number of points you'll earn and the amount of money you'll need to spend will vary by credit card, but you'll usually need to meet a minimum spending requirement within a set timespan.