I'm focusing on a slider where I'll show some contents(featured image and excerpt) drawn from both posts and pages. I wish to filter the posts/pages with a custom area known as 'slider'!

Therefore if you will find some pages and posts with 'slider' custom area only individuals can look within the slider. Is it feasible in wordpress? If it's then how?

Some guidance is going to be appreciated!

What you are searching for may be the class known as WP_Query, has an in depth explanation within the Codex. Have a look in the post_type argument which accepts an assortment, thus you are able to provide a array( 'post', 'page' ) or other publish types that you would like to fetch.

The meta fetch might be completed in two ways, either through the new meta_query argument (from three.1 let's start In my opinion) or meta_key and meta_value that are deprecated since 3.1.

Here is a rough example (haven't checked if the works):

$sider_posts = new WP_Query( array(
    'post_type' => array( 'post', 'page' ),
    'meta_query' => array(
        array(
            'key' => 'slider',
            'value' => 'yes',
            'compare' => '='
        )
    )
) );

while ( $slider_posts->have_posts() ) {
    $slider_posts->the_posts();

    // output the slide here
}

Hope which makes sense. Cheers!

~ K