, like English, is a tricky language full of exceptions. However, unlike the indiscriminately-appropriating and back-formating exceptional exceptions of English, French's exceptions can themselves often be grouped, clustered and categorized into loose collections to whom the general rules fail to apply in the same way
DR and MRS P VANDERTRAMP, when considered as an acronym-mnemonic, are one of these collections*. Each capital letter stands for a verb which pairs with the verb être (to be) when conjugating into the passé composé tense, rather than Francais' typical passé composé verb behavior of pairing with avoir (to have):
(The spaces between DR, MRS, P, and VANDERTRAMP do not have any sorting meaning unto themselves - they are merely aids at massaging the information into the supposedly helpful and memorable configuration which is the name of this node.)
Another, more phonetic rhyming mnemonic exists for remembering the être-conjugators, which omits passer for reasons of multiple-word-meaning. (Passer can conjugate into passé composé with avoir, but means something different there from its usual meaning.):
Entré, rentré, arrivé,
Resté, monté, né, allé,
Tombé, mort, et retourné
(These are words we all can say)
Parti, sorti, descendu,
Revenu, devenu, et venu
(These verbs with être are "conjugués"
When in the passé composé.)
One final complicating aspect of these verbs' conjugation into this tense is that their past participle must accord with both the gender and plurality of their subject. This is not so tricky: group-pronouns like nous, vous, et ils (us, y'all, and those fellas) get an -s added to the end of their participles; female subjects like elle (her) and particular applications of je and tu (I and you) append an -e ... groups of ladies (elles and occasional man-free values of nous and vous) get both with the attachment of an -es suffix.
(An example? While I, as a singular male, might translate "I fell" to "Je suis tombé", if the same fate were to befall jessicapierce and knifegirl the words for them would look more like "Elles sont tombées" - where the first word of each translated sentence is the appropriate pronoun, the middle word is the form of être that accords to the subject, and the third word the past participle adjusted appropriately for still further accordance. Why is all this according necessary? As far as I can tell, the French just like to be able to ignore every other word in a sentence and still be able to tell what the situation is - what's happening to who - at a glance.)
* One more aside: what is it that all the VANDERTRAMP verbs have in common? Eyeballing the list, it seems to me that if you're willing to somewhat spiritually accept birth and death as belonging, they all have something to do with changes in location(al relationships) - a vast array of comings, goings, coming backs, going ups, and not goings anywhere. Devenir is the only stickler, being included perhaps only through some brain-boggling considering that when you become something you actually disappear and something like you but with the new quality takes your place. These sorts of semantic leaps are perhaps best excluded from grammars and left to metaphysics 8)...
mkb notes (and likely far more apropos of anything) "the other common thread is that many verbs conjugated with être are intransitive." I'm glad that we got to the bottom of that one.