The word "ghost" in English mainly refers to the apparition of a dead person, or is used to describe a "ghostly" state, but in Chinese, gui mostly has nothing to do with the dead or undead, when used in everyday terms.
Other than the commonly known and somewhat derogatory term for Japanese "日本鬼子" and foreigners "洋鬼子", "ghost" can be used in a variety of ways, such as to refer to someone with an addiction for example. An alcoholic is a 酒鬼 ("alcohol ghost"), a smoker is a 烟鬼 ("tobacco ghost"), someone addicted to gambling is a 赌鬼 ("gamble ghost"), and a pervert who takes sexual advantage of women at any and every opportunity is a 色鬼 ("lust ghost"). Similarly, a food lover could be teased as a 好吃鬼 ("eating loving ghost"), and a lazy person would be called 懒鬼 ("lazy ghost").
见鬼了 (jiàn guǐ le) or 碰到鬼了 (pèng dào guǐ le) do not actually mean that a person saw a ghost or ran into a ghost, but rather is an exclamation for something out of the ordinary happening. For instance, if I misplace my glasses (as I often do), I could exclaim "真是见鬼了，我记得刚刚还在桌上的！" (literally "truly saw a ghost, I remember it being on the desk just now!). Or if something you bought yesterday went on sale today, you could say: "碰鬼了！昨天才原价买的今天就打折！" ("Holy crap! Bought it yesterday at original price and today it's on sale!")
鬼画符's (guǐ huà fú)literal meaning is "ghost drawn talisman" but it has nothing to do with spirits or Taoist talismans. It's most common colloquial usage pertains to handwriting–really bad illegible handwriting to be more specific. When your doctor hands you a prescription, you may complain to your friends: "他写得鬼画符一样，谁看的懂啊" ("His handwriting is terrible, who would be able to read it")?
In contrast to "God only knows" and "heaven knows", Chinese usually put the all-knowing ghost into the equation. "Only God would know what happened" turns into "Only ghost would know what happened" 鬼才知道发生了什么.
鬼话连篇 (guǐhuà liánpiān) seemingly describe a string of ghost words but, in fact, refers to lies. You may find someone telling you "不要相信他的话，他一直都鬼话连篇，出轨好几次了" ("don't believe his words, he has always been full of bullshit, he has cheated several times already).
What is a "dead ghost"? Sometimes in Chinese, it is an endearing playful nickname. Your middle-aged neighbor might complain to her friends "我家那个死鬼又不愿意洗碗了" (my "dead ghost" – husband – is refusing to wash the dishes again). She may also shout to her husband playing chess outside "死鬼，回家吃饭" (Dead ghost, come home and eat). You may also see a girl pouting (classic sajiao) after her lover kisses her, "you dead ghost, why'd you kiss me!" (你个死鬼，为什么亲我!)
Finally, we leave you with a chengyu that almost no Chinese is able to write, and few can pronounce: 魑魅魍魉 (chī mèi wǎng liǎng). Composed of four characters with the "ghost" 鬼 radical, it is a term for all kinds of ghosts, devils and monsters, as well as bad people. Try showing it to a Chinese speaker, and see what reaction you get!