contrast.bien-bueno
Bien vs bueno
Bien is an adverb (habla bien, estoy bien); bueno an adjective agreeing with nouns (un buen trabajo, una comida buena). Está bien = it's OK/fine.
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Explanation
This is the adverb-versus-adjective distinction. Bien is an adverb: it describes how an action happens and never changes form, habla bien ('speaks well'), todo salió bien ('it all went well'), me parece bien ('sounds good to me'). Bueno is an adjective: it describes a noun and agrees with it in gender and number, un buen trabajo, una buena idea, comida buena.
So the verb decides which one you need. Actions and the verb estar take bien: cocina bien, estoy bien ('I'm fine'). Nouns and the verb ser take bueno: es buena gente ('they're good people'), ser bueno ('to be good or kind'). A common trap is the apocope: before a masculine singular noun, bueno shortens to buen, un buen día, un buen ejemplo.
Two everyday extras: está bien is the all-purpose 'it's OK / fine / agreed'. And in much of Latin America bien colloquially intensifies an adjective (está bien caro ('it's really expensive')) while bueno on its own works as a discourse opener, roughly 'well…' or 'OK…': Bueno, empecemos.
Examples
The team works well; it's a good team.
Region: global
OK, we'll do it your way.
Region: global
OK, if that's fine by you, we'll leave it like that.