'Celtic' languages are considered to be descended from Indo-European, of which Latin, Greek, Germanic and Sanskrit, to name a few, are also connected.
The Celtic languages of the British Isles can be categorised into two groups known as Goidelic
(Q-Celtic) and Brythonic (P-Celtic).
Goidelic is Q-Celtic because it retains the Indo-European 'q' sound by changing it into a hard 'c' or aspirated 'ch'. In the British Isles this is characterised by Gaelic languages, Irish and Scottish and Manx.
Brythonic languages fall in the P-Celtic chategory because they changed the 'q' sound into a 'p'. These originally included Gaulish and Old British but are now characterised by Welsh, Cornish and Breton.