보고 싶다, 그립다 (formal non-polite) 보고 싶어, 그리워 (informal / casual) 보고 싶어요, 그리워요 (informal polite)
그리워: I miss you. To miss a non-person. Mostly used when you miss something or the past that happened relatively long time ago or someone who haven't seen for a while. For example, when you're sick for home. You miss home.
보고 싶어: Literally means I want to see you but it can be mean I miss you. To miss a person. It is used when you miss someone regardless of how long you haven't seen them.