Magento Block Over ride

Here i am going to over ride catalog layer view  [Mage_Catalog_Block_Layer_View]
Config.xml file need to same as that of below.
<?xml version="1.0"?>
<config>
    <modules>
        <Modulename_Catalog>
            <version>0.1.0</version>
        </Modulename_Catalog>
    </modules>
    <global>
        <blocks>
            <modulename_catalog>
                <class>Modulename_Catalog_Block</class>
            </modulename_catalog>
            <catalog>
                <rewrite>
                    <layer_view>Modulename_Catalog_Block_Layer_View</layer_view>
                </rewrite>
            </catalog>
        </blocks>
    </global>
</config>

Then you need to create a class file [Class Modulename_Catalog_Block_Layer_View]  with the function you need to over ride .
No need to change the function name because the system and the config will automatically override the previous one.

Using the above same way you can rewrite models also.