Hi , all India rank stands for the rank secured consisting of students of all categories who appeared for the can whereas category rank means the rank secured by you  among all the candidates that appeared in exam from your category alone . For eg if your all India rank is 3582 then it means 3582 students have secured higher rank than you irrespective of their categories and if your category rank is 58 and you belong to general category then it means that 57 students belonging to general category alone have secured higher rank than you

Hope this helps