What's the difference between 'ser' and 'estar'?

The first important thing to remember is that there is no equivalent in Enlgish, as both verbs mean 'to be'. They can also not be used interchangeably, and both have a slightly different meaning. I think the best way to explain is with an example: 'estou doente' vs 'sou doente'. These both mean 'I am ill', but the first would refer to a temporary illness, like the flu, whereas the second would refer to a lifelong condition or even a terminal illness. So it co0mes down to a matter of permanence. 'Ser' is used to refer to things that aren't subject to change. 'Ele é gentil' - here the person we're speaking about is a kind person, and that is a fundamental part of their personality. If we used 'estar' instead, it would be the case that the person is being kind presently, but they aren't always kind. 

In addition to all of this, it's important to not forget about the verb 'ficar'. This verb is used to describe the location of buildings and places, e.g. o banco fica ao lado de a loja.