Difference between SER and ESTAR
Use SER for defining attributes of things or people (such as being tall, being big, being good-looking).
Use ESTAR for generally non-permanent statuses or conditions of things or people (such as being sad, being broken, or being tired), regardless of whether they are temporary or not in a particular case, and for location (also regardless of whether it is temporary or not).
Ser vs. Estar
Tip: For beginners, it’s a good idea to think of them in terms of “permanent” and “temporary,” but always understanding that this is a simplified way of looking at them. As you progress, it’s best to think of SER as a verb for IDENTITY and ESTAR as a verb for STATUS, CONDITION or LOCATION.
Using them like this you’ll pretty much always get it right. Sometimes, however, with certain adjectives, you can choose to use either SER or ESTAR depending on what you mean. For example, with skinny (delgado/a), you can either say someone is skinny using SER (meaning it’s a defining quality) or using ESTAR (meaning that the person is looking skinny these days). Some adjectives allow this, but it is kind of random, so the best way to learn them is through exposure (by reading a lot and talking to a lot of people in Spanish!).
Lastly, here is a quiz to test your knowledge of SER and ESTAR.