In answer to my question on the difference between "gyūdon" and "gyūmeshi" I learned that "meshi" just means cooked rice. But I thought "gohan" meant cooked rice, so please, what is the difference?
I do already know that "go-" is an honorific prefix and I think I've been told that "gohan" can sometimes be used as a general word for food. But is that all there is to it?
Observe:
飯 meshi
ご飯 gohan
They both mean the same thing, which is (cooked) rice and/or meal. Since rice is an essential part of Japanese cuisine, the two meanings very much overlap. As you said, ご〜 go- is an honorific prefix, which makes ご飯 gohan the politer alternative used in more polite speech.
昼飯 hirumeshi
昼ご飯 hirugohan
Both mean "lunch" (lit. "midday meal/rice"), but their usage differs based on politeness:
昼飯を食ってくる hirumeshi o kuttekuru - "I'll go grab a bite" (colloquial)
昼ご飯を食べてきます hirugohan o tabetekimasu - "I will go out for lunch" (polite)
(translations roughly equated to English usage)
Note that both 飯 and 食 are the same, yet the pronunciations differ widely for different politeness levels. Most words that can be formed with meshi can also be expressed using gohan.
Sometimes as part of compound words the politeness plays less of a role though and it's just the origin of the word that decides the reading:
焼き飯 yakimeshi - fried rice, Japanese word
炒飯 or チャーハン chāhan - fried rice, Chinese word
There's really no difference other than politeness. But politeness is a huge difference in Japanese. For instance, if we take it to the extreme, saying that there's no difference between あなた and 貴様 in Japanese is like saying there's no difference between "Thank you" and "Go to hell." in English. :)
飯 is not as outrageously impolite as 貴様, but in some situations it would feel quite rough. The verb 食う, on the other hand, is probably more impolite than 飯 and I would avoid using it until you feel comfortable enough with the language. 食べる is neutral enough to be used in all informal situations.
One difference other than politeness is when you use with 食 (eat)
ご飯を食べる (gohan wo taberu)
めしを食う (meshi wo kuu)
and めし is written with hiragana most of the time as far as I noticed.
ご飯{はん} is the 美化語{びかご} version of 飯{めし}, i.e. a beautified version. Usually 美化語 has the form of お／ご+the unbeautified version, but ご飯 is an exception. Another exception is 腹{はら}→おなか, belly. はん and なか do not exist by themselves (with those meanings).
Please remember that not all お／ご+noun are 美化語, some are 尊敬語{そんけいご}, respectful language. E.g. お車{くるま}, car. Thus you can say おなか about your own belly, but not お車 about your own car.
gohan can mean cooked rice as well as a meal, since traditional japanese/chinese meals consisted mainly of rice.
Meshi can also mean food in addtion to rice. ひるめし I've heard used for lunch.
One difference is that gohan is kango (Chinese word), whereas meshi is wago (Japanese word).
