a). Enclosed from left, top and right sides, like the picture shows:
For example: "风" and "同".
风 (fēng, wind)

同 (tóng, same)

b). Enclosed from top, left and bottom sides, liek the picture shows:
For example: "区" and "医".
区 (qū, district)

医 (yī, medical)

c). Enclosed from left, bottom and right sides, like the picture shows:
For example: "画 (huà, means picture)"
3). Enclosed on four sides, like the picutre shows: