I'm not sure of the difference, I'll leave it to an native speaker. But the root of معلم is ع ل م or 3-L-M, which has a basic meaning of "know" or "knowledge". Mu3allim / معلم means a person who causes someone else to know, so, a teacher. The mu- is actually a prefix that goes with this form.
They appear to be case endings which aren’t written out in the script (except some texts which mark all vowels due to their religious importance and/or for teaching purposes). In fully vocalised texts this sentence would be أَنتِ مُعَلِّمَةٌ ذَكِيِّةٌ يَا كَرِي. (The ٌ is pronounced “un”, and since any ending causes ة to be pronounced as ت, this makes ةٌ into “tun”)
The case endings apparently are not pronounced in the various colloquial variants of Arabic, and even in formal Standard Arabic the last word in front of a pause has its case endings omitted (that’s why in this sentence, the computer voice reads مُعَلِّمَةٌ ذَكِيَّةٌ, because there is something else following ذَكِيَّة, but in other sentences where there is nothing following it will read مُعَلِّمَةٌ ذَكِيَّة without the -un on the last word.
As for the meaning, the -u- marks nominative case (i.e. it signals that we’re talking about the subject of the sentence) and the -n tells you that the noun is indefinite. So “a teacher” is مُعَلِّمٌ (mu3allim-un), but “the teacher” is اَلْمُعَلِّمُ (al-mu3allim-u).