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If you see a friend in a photo, you could say: ‘I know her!’ However, next to your friend in the picture is another man.
You have seen this man before but only once. You don’t remember his name, and you haven’t spoken to him much. You can say ‘I recognise him.’
To ‘know’ somebody usually means you have experience with them and have spent time with them.
To ‘recognise’ somebody means you have a memory of them but haven’t met them personally, or have only met them briefly and don’t remember them well.
For example: “I don’t know Obama but I would definitely recognise him if I saw him, or if I heard his voice.”
We must be careful.
Sometimes when people say ‘know,’ they actually mean ‘know about’. When you are talking to your friends, one of them might mention a famous singer in a popular band. You don’t know her name but then your friend describes her. You listen and remember and can then say “Oh yeah! I know her.”
Do you know her? Have you met her and talked to her? You probably haven’t. But you do ‘know about her,’ you do ‘know who he is talking about.’
Finally, ‘recognise’ also comes along with an alternate meaning. Something more like: ‘I have noticed this and I respect/understand it’. This is a more formal usage of the word.
For example, in a strong discussion at work, you might say: “I recognise that building a new office would give the team a more productive environment to work in, but at the moment we simply don’t have enough funds to make such large changes.”
This doesn’t mean that you remember something that somebody said before. It means that you have heard what they said, you understand/respect/agree with what they said, but you still want to make a different point.