There are twelve musical alphabets in music. They are C, D, E, F, G, A, B, C#/Db, D#/Eb, F#/Gb, G#/Ab, A#/Bb. Note B and E have no sharp and C and F have no flats. 
In piano the white keys are natural notes and the black keys are sharps or flats.

Okay enough, What are sharp and flats?

If you see, there are seven notes that have no any sign other than the English alphabets. But there are five notes which have # or b sign behind them. You may know that every notes has its own specific frequency. If we look closely, the note C has its own frequency and note D has its own frequency. But there is another note which has frequency in between the frequency of C and D note. That note is neither C nor D. It is represented as C# or Db.  
So, If we consider F#, then its frequency lies in between the frequency of F and G. But that note is neither F nor G. 
A# and B♭ is the same note. F# and G♭ is also the same note. # sign means it’s frequency is higher than A and ♭ means it’s frequency lower than B note. 

What is the relation between the musical notes?

Before that we need the concept of octave. In music, an octave is the interval between one musical pitch and another with double its frequency. In piano, you may see the C note after every 7th white key, but all have different pitch and sound and yet called C. Also in guitar you can find the octave of any note, after 12 frets. 
In simple language, an octaves are the same note with different frequency.
Let us consider a musical graph which goes from left to right. Left side of the graph contains lower notes and right side contains higher notes. The notes go higher while moving from left to right. If we go like that, in regular interval same notes with different frequency repeats. That is octave.
If we divide that line in between adjacent octave in 12 equal spaces. Each space represents each note. So there are twelve different notes and the distance between them is equal.
So the distance between C to C# is same as distance between D# to E. The distance between B to C is also the same with C to C# (not with C to D).
In music distance between each note is call half-step. And the distance between two adjacent note is call whole step. So one whole-step is equal to two half-steps. So one whole-step after C is D, one whole step after B is C# and the one whole step after E is F#. Also one half-step after B is C, one half-step after D is D# and one half-step after E is F.