関係 (kankei) refers to the connection between two or more things, people, or events. It can be a personal relationship, a business connection, or how things are related causally. It's a very common and broad term. For example, '彼とは良い関係を築いている。' (I have a good relationship with him.)

関係 (かんけい) は人や物事のつながり。友人関係や仕事などで使います。例：彼との関係は良好だ。