A baritone horn looks somewhat like a miniaturized tuba. A bass trombone is still a trombone, but slightly larger. It also has an F trigger, which opens up more tubing near the bell of the trombone and changes the note. This allows players to reach the range from a low E flat to a low B, which is physically impossible on a tenor trombone. Some bass trombones also include another trigger, G flat, which can be opened separately or in combination with the F trigger.

Trombone are different for everyone so I recommend you go to a band shop and just try out their trombone and see what brand you like best.

about the same as a dictionary or 1 kg

No. The trigger enable you to hit #7 position without moving the slide. There is plenty of pro players who do not use a trigger trombone.

Take band in school.