女士(nǚ shì)
It’s “lady” or “madam” in English. When referring to an adult woman in formal or professional settings, this address term is often used.
女士 can be used to address all adult women regardless their ages or marital situations. It is neutral and shows respect.
e.g.
女士，您请这边走。(Nǚshì, nín qǐng zhè biān zǒu.) Madam, please come this way.
美丽的女士，我能请您跳个舞吗？(Měilì de nǚshì, wǒ néng qǐng nín tiào gè wǔ ma?) Beautiful lady, may I have this dance with you?
张女士，久等了，好久不见！欢迎您来上海。(Zhāng nǚshì, jiǔ děng le, hǎojiǔ bùjiàn! Huānyíng nín lái Shànghǎi.) Ms. Zhang, you’ve been waiting, long time no see! Welcome to Shanghai.
小姐(xiǎo jiě)
It’s “miss” in English. 小姐 is often used to address younger and unmarried women.
However, this term’s use can vary regionally. In some parts of the Chinese-speaking world, 小姐 has developed a derogatory connotation, sometimes being associated with prostitutes. But if you add their surnames before it, there is no misunderstanding at all.
e.g.
小姐，你的手机忘拿了。(Xiǎojiě, nǐ de shǒujī wàng ná le.) Miss, you forgot your phone.
李小姐，你的外卖送到了。(Lǐ Xiǎojiě, nǐ de wàimài sòng dào le.) Miss Li, your takeout has arrived.
王小姐，幸会。我是这次旅行的导游。(Wáng Xiǎojiě, xìnghuì. Wǒ shì zhè cì lǚxíng de dǎoyóu.) Miss Wang, nice to meet you. I am the tour guide for this trip.
看！那几个女的看起来像小姐，你觉得呢？(Kàn! Nà jǐ gè nǚ de kàn qǐ lái xiàng xiǎojiě, nǐ juéde ne?) Look! Those women look like prostitutes, what do you think?