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Solidity docs explains the concept of Inheritance well.

Inheritance

Solidity supports multiple inheritance by copying code including polymorphism.

All function calls are virtual, which means that the most derived function is called, except when the contract name is explicitly given.

When a contract inherits from multiple contracts, only a single contract is created on the blockchain, and the code from all the base contracts is copied into the created contract.

The general inheritance system is very similar to Python’s, especially concerning multiple inheritance.

Details are given in the following example.

pragma solidity ^0.4.16;

contract owned {
    function owned() { owner = msg.sender; }
    address owner;
}

// Use `is` to derive from another contract. Derived
// contracts can access all non-private members including
// internal functions and state variables. These cannot be
// accessed externally via `this`, though.
contract mortal is owned {
    function kill() {
        if (msg.sender == owner) selfdestruct(owner);
    }
}

// These abstract contracts are only provided to make the
// interface known to the compiler. Note the function
// without body. If a contract does not implement all
// functions it can only be used as an interface.
contract Config {
    function lookup(uint id) public returns (address adr);
}

contract NameReg {
    function register(bytes32 name) public;
    function unregister() public;
 }

// Multiple inheritance is possible. Note that `owned` is
// also a base class of `mortal`, yet there is only a single
// instance of `owned` (as for virtual inheritance in C++).
contract named is owned, mortal {
    function named(bytes32 name) {
        Config config = Config(0xD5f9D8D94886E70b06E474c3fB14Fd43E2f23970);
        NameReg(config.lookup(1)).register(name);
    }

    // Functions can be overridden by another function with the same name and
    // the same number/types of inputs.  If the overriding function has different
    // types of output parameters, that causes an error.
    // Both local and message-based function calls take these overrides
    // into account.
    function kill() public {
        if (msg.sender == owner) {
            Config config = Config(0xD5f9D8D94886E70b06E474c3fB14Fd43E2f23970);
            NameReg(config.lookup(1)).unregister();
            // It is still possible to call a specific
            // overridden function.
            mortal.kill();
        }
    }
}

// If a constructor takes an argument, it needs to be
// provided in the header (or modifier-invocation-style at
// the constructor of the derived contract (see below)).
contract PriceFeed is owned, mortal, named("GoldFeed") {
   function updateInfo(uint newInfo) public {
      if (msg.sender == owner) info = newInfo;
   }

   function get() public view returns(uint r) { return info; }

   uint info;
}

Note that above, we call mortal.kill() to “forward” the destruction request. The way this is done is problematic, as seen in the following example:

pragma solidity ^0.4.0;

contract owned {
    function owned() public { owner = msg.sender; }
    address owner;
}

contract mortal is owned {
    function kill() public {
        if (msg.sender == owner) selfdestruct(owner);
    }
}

contract Base1 is mortal {
    function kill() public { /* do cleanup 1 */ mortal.kill(); }
}

contract Base2 is mortal {
    function kill() public { /* do cleanup 2 */ mortal.kill(); }
}

contract Final is Base1, Base2 {
}

A call to Final.kill() will call Base2.kill as the most derived override, but this function will bypass Base1.kill, basically because it does not even know about Base1. The way around this is to use super:

pragma solidity ^0.4.0;

contract owned {
    function owned() public { owner = msg.sender; }
    address owner;
}

contract mortal is owned {
    function kill() public {
        if (msg.sender == owner) selfdestruct(owner);
    }
}

contract Base1 is mortal {
    function kill() public { /* do cleanup 1 */ super.kill(); }
}


contract Base2 is mortal {
    function kill() public { /* do cleanup 2 */ super.kill(); }
}

contract Final is Base1, Base2 {
}

If Base2 calls a function of super, it does not simply call this function on one of its base contracts. Rather, it calls this function on the next base contract in the final inheritance graph, so it will call Base1.kill() (note that the final inheritance sequence is – starting with the most derived contract: Final, Base2, Base1, mortal, owned). The actual function that is called when using super is not known in the context of the class where it is used, although its type is known. This is similar for ordinary virtual method lookup.