Polite style and plain style
です・ます ↔ plain form (だ, 行く, 高い…)
The polite style (です・ます) is used with strangers, with superiors and in formal situations. The plain style uses the plain forms of verbs and adjectives (行く, 行かない, 行った, 高い, 静かだ) and belongs to family and friends.
Examples
あした 東京へ 行く?…うん、 行く。
Are you going to Tokyo tomorrow? …Yes, I am.