Tum is used mostly between friends and other people of same age its informal translation of you while aap is formal one it is used towards aged people your teachers and any elder person ,just to show some respect

Mr.Umair, I welcome you on italki.com. Aap, is a common word in Urdu, to address, specially Adults. Tum, You can use this word to address a child or anybody younger. Please feel free to ask more Questions.

My understanding is that 'aap' is a much more formal and polite version . You can use it with people you've just met , or with adults or if you just wish to be more polite . Tum, on the other hand is used more often among friends and people you are familiar with . So, an informal version of the same word .

Hello Umair! My name is Ikram & I'm a student. I think I can help you in learning Urdu. Well I'm learning English, I hope we can help each other and can practice together. Give me a reply if u r interested. Thanks