Since chocolate is plant-based (made from cacao beans), it can be vegan.
Some contains dairy products, which is obviously not vegan. Apart from dairy products, what other common ingredients in chocolate bars should be avoided?
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The easiest way to know is to scan the ingredients list - good quality standard chocolates don't tend to have a large amount of ingredients so it shouldn't take long. Looking at the allergen section for "contains Milk" is a pretty quick way of discovering if it's suitable, too.
In Europe we have E Numbers, some of which aren't Vegan and may be present in less "plain" types of Chocolate. Probably better to Google those if you come across something you might want to get, but here's a list of non-Vegan ones.