ほど〜ない

"ほど〜ない" is used to compare two things. In many cases, both have a certain degree of a characteristic, but one has a higher degree than the other.

 今年の冬は去年ほど寒くないです。
ことしのふゆはきょねんほどさむくないです。
This year's winter is not as cold as last year.

In this case, we are comparing the degree of coldness of winter each year, acknowledging that both years are cold to some extent.

 山田さんは、佐藤さんほど英語が上手ではないです。
やまださんは、さとうさんほどえいごがじょうずではないです。
Yamada-san is not as fluent in English as Sato-san.

Here, the impression given is that both Yamada-san and Sato-san are reasonably good at English, but Sato-san is better.

"より〜だ" is also used for comparison, but this is a more straightforward comparison between two things.

 冬は秋より寒いです。
ふゆはあきよりさむいです。
Winter is colder than autumn.

佐藤さんは山田さんより英語が上手です。
さとうさんはやまださんよりえいごがじょうずです。
Sato-san is more fluent in English than Yamada-san.