Truecaller is providing free service of caller ID to the Smartphone users. Truecaller secretly steals your contacts and uploading to their servers. So whenever you got call from unknown number they fetch it with the server and gives you the name of contact.
There is a chance that Truecaller might sell the contact they have steal from your phone with the high rate. I read somewhere that 1 one real contact is sold in 1$. So if you have 200 contacts in your smart phone then it is 200$ worth.
Sometimes truecaller is very useful app but the worst part is they are stealing your contacts. So I recommend make your contact private before installing this app.(Android Marshmallow provides you this feature) So don’t give truecaller the permision to see your contacts.
Overall its a useful app but at the cost of your contacts.