This pattern compares two things and says that the first is less than the second with respect to the predicate. It is like saying 'A is not as [adjective] as B'. The thing being compared to (the standard) is marked by ほど, which literally means 'to the extent of'. So 春は秋ほど寒くない means 'Spring is not cold to the extent that autumn is' – i.e., spring is less cold than autumn. You can use it with verbs too: 彼は私ほど食べない means he doesn't eat as much as I do. The negative can be in the form of ない, ません, or ではありません, and the predicate can be an adjective or verb. Note that the standard (the one with ほど) is the one that is greater in degree. If you say 私は弟ほど背が高くない, you are shorter than your brother. The pattern is common in both conversation and writing. It is not rude or overly formal, but sounds natural in everyday speech when using ない or ません. Be careful not to confuse it with ほど...ない which means 'no more than' or 'only' in some contexts – here it is purely comparative.