UML relations notation

IS-A (Inheritance) : extends / implements(realization)

class Laptop {
   // Code for Laptop class goes here.
}

interface Formatable {
   // Members of Interface.
}

class Dell extends Laptop implements Formatable {
   // More code related to Dell goes here.
   // Dell class will inherit all accessible members of Laptop class.
   // Dell IS-A Laptop.
   // And Dells class also implements all method of Formatable interface, since
   // Dell is not an abstract class.
   // so Dell IS-A Formatable.
}

Has-A (Association) :

class HardDisk {
   public void writeData(String data) {
      System.out.println("Data is being written : " + data);
   }
}

class UseDell {
   // segate is referece of HardDisk class in UseDell class.
   // So, UseDell Has-A HardDisk
   HardDisk segate = new HardDisk();
   public void save (String data) {
      segate.writeData(data);
   }
}

Aggregation :

Consider above example : HardDisk class and UseDell class.
In the example, UseDell HAS-A HardDisk.

If Laptop stops working somehow, you can remove harddisk from that and connect to other laptop, This is Aggregation.

Composition :

class House {
   Kitchen kitchen = new Kitchen();
   // More code for House class.
}

class Kitchen {
   // code of Kitchen class.
}

If House gets destroyed Kitchen also will be destroyed with that House, This is composition.
In composition reference class (Kitchen) can not exist if container class (House) gets destroyed.

reference : 

Good definition of relations (java)