Is passing the session id as url parameter really insecure?
While it's not inherently insecure, it can be a problem unless the code is very well-designed.
Let's say I visit my favorite forum. It logs me in and appends my session ID to the URL in every request. I find a particularly interesting topic, and copy & paste the URL into an instant message to my friend.
Unless the application has taken steps to ensure that there's some form of validation on the session ID, the friend that clicked that link may inherit my session, and then would be able to do anything I can do, as me.
By storing session identifiers in cookies, you completely eliminate the link sharing problem.
There's a variation on this theme called session fixation, which involves an intentional sharing of a session identifier for malicious purposes. The linked Wikipedia article goes into depth about how this attack works and how it differs from unintentional sharing of the session identifier.
Why are cookies more secure?
Cookies can be more secure here, because they aren't something that normal users can copy & paste, or even view and modify. They're a much safer default.
What possibilities does an attacker have for both options?
Neither of these methods is secure from man-in-the-middle attacks over unencrypted communication. Browser addons, spyware and other client-side nasties can also spy on both methods of storing session identifiers.
In both cases, server-side validation that the client that claims to own a session ID is best practice. What this validation is composed of is up for debate. Keep in mind that users behind corporate proxies may hop between IP addresses between requests, so locking a session to an IP address may accidentally alienate people. The session fixation article mentions a few other helpful alternatives.