What's the difference between "veux" et "voudrais"?
as title, for example
Je veux un café. Je voudrais un café.
I know the second one is asking for a service in a restaurant, are they in the same meaning but the second is more polite?
May I use the first sentence? In which condition?
"Veux" means "want". Therefore, "Je veux un café" means "I want a coffee". But "voudrais" means "would like" so "Je voudrais un café" means "I would like a coffee". The second one is more polite in a way. The first one sounds like you have a bit of a grudge so I recommend to not use that among our elders or around our friends or important people.