ties = when the line is connecting between 2 same notes of the same pitch, which means if the first note is a Bb and the second is a Bb but one octave lower than it is NOT a tie. You play the first note and then hold it for the joined notes' time values. Basically, don't play the second note, just continue to hold it.
slur = when the line is from one note and connected to another note that is not the same pitch, and there may or may not be notes under the slur. Generally, you play all the notes under the slur in legato style (depends on your piece). And most of the time you play from loud to soft (i'm speaking relatively). Like if there were 2 notes connected by slur, most chances you wouldn't play the 2nd note louder than 1st