What is the difference between talking to a saint and talking to a ghost?
The short answer
The Church forbids occult methods of summoning the dead — Ouija boards, séances, mediums — and attempts to manipulate the supernatural for forbidden power or knowledge. Praying to a saint conjures nothing; it honors God's friend and asks for prayer.
What is forbidden
The Church forbids methods used to summon the departed or other spirits — Ouija boards, crystal balls, séances, mediums — and attempts to manipulate the supernatural for forbidden power or knowledge (CCC 2116). It does not forbid all communication between this life and the next.
Saul and Samuel
Saul's sin at Endor (1 Samuel 28:7-20) was not his desire to speak with Samuel but the forbidden means. He could have prayed to Samuel for intercession; instead he had a medium conjure him. God may sometimes allow such contact to bring good from evil, but the methods remain unlawful.
Prayer to saints
Praying to saints involves no conjuring and no forbidden knowledge. We honor God's friend and ask for prayer.