Which is slower - andante or adagio ?

Here's an excerpt of the Wikipedia entry on tempo:

Basic tempo markings

By adding an -issimo ending, the word is amplified. By adding an -ino or -etto ending, the word is diminished. The beats per minute (bpm) values are rough approximations.
From slowest to fastest:

• Larghissimo – very, very slow (24 bpm (beats per minute in a 4
  4 time) and under)
• Grave – very slow (25–45 bpm)
• Largo – broadly (40–60 bpm)
• Lento – slowly (45–60 bpm)
• Larghetto – rather broadly (60–66 bpm)
• Adagio – slow and stately (literally, "at ease") (66–76 bpm)
• Adagietto – slower than andante (72–76 bpm)
• Andante – at a walking pace (76–108 bpm)
• Andantino – slightly faster than Andante (although in some cases it can be taken to mean slightly slower than andante) (80–108 bpm)
• Marcia moderato – moderately, in the manner of a march (83–85 bpm)
• Andante moderato – between andante and moderato (thus the name andante moderato) (92–112 bpm)
• Moderato – moderately (108–120 bpm)
• Allegretto – moderately fast (112–120 bpm)
• Allegro moderato – close to but not quite allegro (116–120 bpm)
• Allegro – fast, quickly, and bright (120–168 bpm) (molto allegro is slightly faster than allegro, but always in its range)
• Vivace – lively and fast (168–176 bpm)
• Vivacissimo – very fast and lively (172–176 bpm)
• Allegrissimo or Allegro vivace – very fast (172–176 bpm)
• Presto – very, very fast (168–200 bpm)
• Prestissimo – even faster than Presto (200 bpm and over)

Much more at Wikipedia.