A F# Minor 7 (F#m7) chord contains the notes F#, C#, E, A. It is built from the minor 7th formula 1 - ♭3 - 5 - ♭7, with F# as the root.
Why does Wonderwall's "Em7" actually sound like F#m7?
Guitarists play Wonderwall with a capo on the 2nd fret, fingering the easy open Em7 shape. The capo raises every note a whole step, so the shape that looks like Em7 sounds as F#m7 — its real pitch. On piano or bass, with no capo, you play F#m7 directly.