Where是关系副词,在定语从句中做地点状语时用或者说如果在从句中既不做宾语也不做主语或表语时使用.

eg. This is the shop where I bought my new shoes.

(做状语)

This is the shop which I visited yesterday.(做宾语)

This is the shop which I bought my new shoes in.

(做宾语)

This is the shop which sells shoes.(做主语)