A major interval is an interval between B and another note that is part of the B major scale. In common usage, it only refers to non-perfect intervals -- that is, one that needs a resolution for the music to sound "finished." For instance, B to C sharp is a major second, because C sharp is a note in the major scale, and you cannot end a piece on a second without feeling as though there is more to come. Like all keys, the key of B major has four major intervals: a major second between B and C sharp; a major third between B and D sharp; a major sixth between B and G sharp; and a major seventh between B and A sharp.
When you lower the top note of a major interval one semitone -- or to the next key on the keyboard, whether it be black or white -- you create a minor interval. These intervals are always between B and a key that is not included in the B major scale. For example, B to A natural is a minor seventh, because A natural is not a part of the B major scale, and it is one semitone below A sharp, which creates the major seventh. All of the scale's major intervals can also be made minor, which means that there is a minor second between B and C natural; a minor third between B and D natural; a minor sixth between B and G natural; and a minor seventh between B and A natural.
The top note in a perfect interval is both a part of the scale and has a high degree of consonance, which means that you can end a musical piece on a perfect chord and the music would sound resolved. B major has four of these intervals, including the perfect first between B and itself, also called a unison or a perfect prime; the perfect fourth between B and E; the perfect fifth between B and F sharp; and the perfect eighth between B and the next highest B, also called an octave.
When the top note in a major or perfect interval is raised one semitone, the interval becomes augmented. Likewise, when the top note in a minor or perfect interval is lowered one semitone, it becomes diminished. Note that major chords cannot become diminished, because lowering their top note one semitone creates a minor interval. Some intervals can be both augmented and diminished, depending on how they were created. For example, the interval between B and F natural is both an augmented fourth -- because the F is one semitone above the E in the perfect fourth -- and a diminished fifth, because it is one semitone below the F sharp that forms the perfect fifth. Besides these two intervals, B major also contains an augmented fifth, between B and G natural.