The following intervals are all major seconds because the two notes are exactly two half steps apart. They skip one pitch in the chromatic scale. On the piano keyboard, these notes are played by keys that have exactly one key in between.
For example, the first interval below is Bb to C. This interval skips B. There is one half step from Bb to B, and a second half step from B to C. Therefore, Bb to C contains two half steps.
A major second is also known as a whole step, which is used (along with half steps) to define scales.
The whole tone scale is a scale consisting of all whole steps. Like the chromatic scale, every interval in the whole tone scale is the same size.
The following is an example of two different whole tone scales. Notice that each scale contains only 7 notes, so at some point we skip a letter name (line or space on the staff).
Counting Whole Steps
Like half steps, counting whole steps differs from counting the numeric portion of the interval. When you count whole steps, the first whole step occurs when the distance of a whole step has been travelled.
So, C to D is a major second because I count C as 1 and D as 2 to find the numeric portion of the interval. C to D is 1 whole step because the distance between C and D is a single whole step.