What's the difference between 'avoir' and 'être' verbs?

When conjugating a verb (i.e. when we work out that the verb avoir is in these forms: j'ai, tu as, il/elle/on a, nous avons, vous avez, ils ont, that is the same thing as conjugating the verb) in the perfect tense/passé composé, some verbs take 'avoir' e.g. J'ai lu (lire) whilst some verbs take 'être' e.g. Je suis allé(e) (aller). In the majority of cases the verb will take 'avoir', but in some verbs, mainly verbs which express movement or change of state (Je suis mort(e), elle est devenue, il est tombé etc.) take 'être'. A good set of 'être' verbs to start out with are the DR and MRS Vandetramp verbs.