For a native English speaker, they're completely different vowels. Food /u/ is a long, tense vowel, while good /ʊ/ is a short, lax vowel pronounced in a less extreme way.
To relate this to some of your recent posts, this distinction is about as different as Russian soft/hard vowels (that is, it's really the following vowels that differ, rather than the consonants themselves).
If you don't distinguish them, you will sound foreign but probably will still be understood in context, in the same way that a foreign Russian speaker would be understood if they don't distinguish soft/hard sounds consistently.
To give another comparison, this is actually the same distinction as beat /i/ (long, tense) and bit /ɪ/ (short, lax, more in the center of the mouth). It's a relevant and clear contrast for native speakers, but non-native speakers can still be understood. It's also a typical example of a joke about learning English, where you want to be sure not to pronounce "sheet" or "beach" with /ɪ/!
There are also exact minimal pairs you can use, rather than these not quite equivalent examples. Here are some:
who'd vs. hood
shoed/shooed vs. should
cooed vs. could
suit vs. soot
fool vs. full
etc.
(And not that I should need to say it, but never rely on English spelling for anything except a rough guess about how a word might be pronounced. For a variety of historical reasons, including original dialectal variation and then later merging of spellings, along with other shifts, and just general inconsistency, forms like "oo" may indicate a variety of pronunciations.)
[Note: I'm writing this from the perspective of General American English. The specific pronunciation may vary in other dialects, although I don't know of any English variety that does not distinguish these sounds.]
