Synecdoche vs Metonymy

Synecdoche: 

Sometimes the material an item is made of can be used as synecdoche in place of the actual object. When a sword is referred to as 'steel,' for instance, this is synecdoche, since the entire sword is probably not made of steel. Moreover, the sword could be made of another metal altogether, but the historical connection between 'steel' and 'sword' is powerful enough to make it synecdoche nonetheless.

Likewise, if someone asks, 'Are you wearing Kevlar?' you might know from watching enough action movies that this is synecdoche for a bulletproof vest, while using 'plastic' at the grocery store means putting the bill on your credit card because credit cards are made of plastic. As with materials, containers can sometimes come to refer to the objects they contain - another form of synecdoche. As in, 'Nazie drank the cup,' which doesn't literally mean that Nazie swallowed a small cup, but rather that she drank the contents of the cup. Likewise, 'The bartender is giving away the bar,' means that he or she is giving out too many free drinks, which is the stuff the bar contains.

Metonymy

Metonymy is when a thing refers to something else that it's closely associated with, but unlike synecdoche, the part does not have to refer to the whole, or vice versa. Remember when we talked about how 'wheels' was synecdoche for 'car?' Here's the metonym version of the same:

'It was the town's mechanic, not the rich lawyer, who had the nicest ride.'

'Ride' here is a metonym for 'car' because riding is something you do in an automobile, but the 'ride' is not a part of the automobile and therefore does not qualify as synecdoche.

Here's another example: If someone asks you how many plates there are going to be at dinner, what they're really asking you is how many guests are going to show up. Plates are intimately associated with the act of eating, which is what dinner guests typically do, and therefore 'plates' is metonymic for 'dinner guests.' Similarly, if someone tells you 'You have nice kicks,' that's a metonym for shoes, since kicking is something you do with your feet and you wear shoes on your feet. It's not parts referring to wholes (that's synecdoche) but contextual associations linking one word to its meaning in conversation or writing. Technically, synecdoche is actually a very specific kind of metonymy, but synecdoche is a little easier to wrap your head around, and other types of metonymy don't get their own specific categories