The Euphonium and Baritone are closely-related instruments. The most-significant difference is the shape of the bell. The Euphonium's bell points upwards while the baritone typically has a bell bent slightly outward. In addition, some players (typically those that label themselves as "baritones" read in the treble clef, as a transposing instrument. Those that read in the bass clef learn a different set of fingerings that enable them to be a non-transposing instrument. Therefore, a Bb Baritone will play a "C" as "open", while a Euphonium player will play a C as "first finger".
The B-flat fingering series matches the trumpet. As such, any young musician switching from trumpet to baritone will find it easier to continue to read treble clef and use the familiar trumpet fingerings, rather than learn a new clef and a new set of fingerings.
Due to its conical nature, it has a rich, mellow tone and lacks the brilliance of the trombone. It blends well with other instruments, particularly the tuba, which it often doubles at the octave. It can be more agile than the trombone, due to its valves.
Often, the tenor sax and Bb baritone parts double one another, while the Euphonium player is capable of reading trombone parts (such as Trombone 3 with the tuba)