"To do it simple, a slur is usually a curved line upon several notes. It just means several notes in one bowstroke like Pixelpoet said. " -javajam
That is absolutely FALSE. Strings very commonly play slurred notes with different bow strokes.
It is necessary many times to play slurred sections of music with different bow strokes because it may be impossible to play all the notes on one bow. For example - an FF section where the player is just sawing away at his instrument and can only take two notes at a time on one bowstroke beause he runs out of bow. Or another example is a change between notes that are impractical on one bow and would have too much portamento. Like jumping an octave - with a slur it might often be taken on a different bow bbecause if played SUL it would be too much of a slide. So a slur in notated music does NOT mean strictly "take it in one bow."