是 (shì) is the chinese equivalent of the English verb "to be".

Sentences like those expressing religion, nationality, etc. use this verb:
For example:
我是中国人 (wǒ shì Zhōng guó rén) - Literal: I am china person - I am chinese.

However, when specifying professions, the verb 当 (dàng) or 做 (zuò) is used:
For example:
我当老师 (wǒ dàng lǎo shī) - I am a teacher
我做老师 (wǒ zuò lǎo shī) - I am a teacher

Outside China the _expression_ 我是老师 is perfectly acceptable. Note that the _expression_s 我当老师 and 我做老师 are considered formal, though they do appear in informal contexts.

The negation of this is made with 不 (bù):
For example:
我不是中国人 (wǒ bú shì Zhōng guó rén) - Literal: I not am china person - I am not chinese

Let us look at a longer sentence using 是 (shì):

我是你的中国老师 (wǒ shì nǐ de Zhōng guó lǎo shī) - Literal: I am your china teacher - I am your chinese teacher