What is the main difference between detect controls and prevent controls ?

The controls designed to prevent an error before it occurs are preventive and controls designed to catch errors after occurrence are detective controls.

i.e Segregation of duties are best example of preventive control and reconciliation or review of operational activities are detective controls.

Preventive: (helps in preventing the risk before its occurence), some examples:

• Authorization and approval procedures; 
• Supervision (assigning, reviewing/approving, guidance, training); 
• Segregation of duties (authorizing, processing, recording, reviewing);
• Controls over access to resources and records

Detective:(helps in uncovering and detecting the risk after its occurence), some example:

• Reviews of operating performances;
• Reconciliations;
• Verifications; 
• Reviews of processes and activities

The controls designed to prevent an error before it occurs are preventive and controls designed to catch errors after occurrence of are detective controls. 

Prevent is more of a "blocker" and Detecti is more  "hunting down"