Function to distinguish pages and posts

I’ve put the the_meta-tag (custom fields) in my index-file of the WordPress installation. But I do not want to display the custom fields on static pages, but only on posts of my blog. I’m searching a function that distinguishes between a page (like impressum e.g.) and a post (like this is my first post, welcome to my blog e.g.). Can someone help me?
Thanks in advance…