The Relationship Between “do re mi” and note letter names?

When the people sing the song, we use do, re, mi, fa, sol, la, ti, si, ect, but some time I hear that the music people say A, B, C, C flat, ect.
Is their relationship or are they two different things?
Do-re-mi-etc. is "sol-fa" or "solfege".
Sol-fa represents a major scale, with Doh being the first note, Re being the second, and so on. I'm sure you can sing that scale.