口 denotes mouth. Characters with 口 relate to mouth, such as acts of mouth 吃, 喝; parts of mouth 嘴, 唇, etc.
囗 denotes encircle. Characters with 囗 relate to encircle, surround, boundary, etc, e.g. 围, 圈.
Tips on learning characters with 口 and 囗:
1. Most of the characters with 口 are formed in the semantic left-phonetic right structure.
2. Most of the characters in fully-enclosed structure with 囗 are formed in the phonetic inside-semantic outside structure.