Think of TCP as a dedicated scheduled UPS/FedEx pickup/dropoff of packages between two locations, while UDP is the equivalent of throwing a postcard in a mailbox.
UPS/FedEx will do their damndest to make sure that the package you mail off gets there, and get it there on time. With the post card, you're lucky if it arrives at all, and it may arrive out of order or late (how many times have you gotten a postcard from someone AFTER they've gotten home from the vacation?)
TCP is as close to a guaranteed delivery protocol as you can get, while UDP is just "best effort".