"Used to" and "would"

They mean the same thing.  One difference is that I would play with my friends implies that something else is coming in the story, while I used to play with my friends is just a statement of past fact.   There is a sense of ongoing time with would.

I would play with my friends every Saturday until it got dark.

Another difference is that used to goes better with verbs that describe ones personal state of mind at the time.  

I used to understand Spanish, but now I have forgotten it all.  You can't say that with would.

I'll add this here too, so it's easier to read.

I see that there are already some answers on this. Just to add a bit more information, this might be useful for you.

'Used to' and 'would' are often interchangeable in English, as is the case in the example that you give. However, their uses are different in certain cases.

'USED TO' - refers to a continuous event in the past or an event that occurred over a period of time.

'WOULD' - refers to a repeated event in the past.

So: 'When I was a child I used to visit my grandparents in London' - the sense is that it's a past event that occurred perhaps over many years.

'When I was a child I would visit my grandparents in London' - the sense is that it's a past event that was repeated many times.

Essentially it means exactly the same as the first example.

However, When I was a child I used to go to school in Belfast. - This is a past event that is repeated over a period of time, so we can say 'used to'.

However, in this sense it isn't a 'repeated event'. The idea of going to school in this sentence is talking about a period of time. So we CAN'T use 'would' in this sentence.

However, we could say 'When I was a child I used to walk to school' or 'When I was a child I would walk to school' as both have the sense of an action that was repeated every day.

With the verb 'would', it is quite common to mention the frequency of the event - 'When I was a child I would walk to school EVERY DAY'.

Hope this helps! Any questions, just ask!

Leigh