A slur is indicated by a curved line that begins on the first note of the slurred group of notes and continues to the last note. A slur is an articulation which connects notes with different pitches. On a bowed instrument, for example, the bow continues in the same direction on a slur, as much as possible. On a wind instrument the note similarly continues without being rearticulated. A singer stays on the same syllable and on a keyboard instrument the performer just tries to join those notes smoothly together.
A tie is also a curved line which joins two or more notes. The difference being that they are the same pitch. In the example below, the second note is a continuation of the first and the length becomes the result of adding the two note values together.
A slur may be hard to distinguish from a phrase mark, which looks like a slur but usually covers a longer passage and really indicates one musical thought. It is like a phrase in spoken language. Some writers, such as Gardner Read in his authoritative text Music Notation, treat the two terms as more or less equivalent. Personally I prefer to distinguish between slurs and phrase marks because the slur relates to articulation and the phrase to the outline of the music. In some pieces I have had to sort out whether notes under a phrase mark should be slurred or not. Trust your sense of the music melody and musical line for this.
