IP stands for internet protocol.IP address is unique string of numbers seperated through periods and this unique address represent computer on network. These ip address are divided into various classes and then classes are divided into blocks and these blocks are sold to isp(internet service providers) of countries due to each ip represent computer on network in sepcific region. When you connect to internet your isp assigned you one specfic ip address which represent your system on internet. Domain are human readable and memorizable strings like google.com facebook.com etc these are so call domain because they are easy to remember by humans rather than ip address which are numbers and difficult to remember. So every domain represent one ip address , when domain name like google.com is entered in browsers its fast of all translated or can resolved into ip address then requests are sent to that server through its ip address.
Useotools Reverse IP Domain Scanner tools is used to find other domains which are using same ip address like your site, for example if you using shared hosting then its clear that other people will be using it to i mean shared hosting in first place mean that one server is shared by a lot of website owners so who are the other owners or can say what other websites or domains are hosted on server on which your sites is hosted to find that Useotools team designed tool called Reverse ip domain scanner. Usage of this tool is simple you just enter your domain name and it will scan it and will tell you that there are or not other domains exists who are using this server.