watch out & Be careful

"Watch out!" 或 "Look out!" 通常是用來作千鈞一髮之際的即時緊急警告，例如有人橫過馬路時不為意有車駛至，會用 "Watch out!" 警告之，亦因為此，只適用於口語（寫得來都撞死了）。
"Be careful" 或 "Beware" 則通常用於事前警告，例如媽媽教小朋友過馬路，過之前便對他們說 "Be careful when you cross the road."

The meaning of the two words are the same. However, "watch out" is less formal. it is usually used in conversation where as "be careful" would be used in sentences.

E.g: You got to "watch out" when crossing the street.
E.g: You go to "be careful" when crossing the street.

Same. no difference. But 'watch out' seems to be used more in conversation:

eg: Watch out ! A car is coming !