Once you click the link:
1. You'll be taken to a page that prompts you to "upgrade your Flash player" or similar.
2. If you agree to this download, the software installs itself.
3. You receive error messages saying you have a virus and need to download a fake antivirus program they call "Fast Anti-Virus 2009". This is the end goal of the scam: to get you to pay $89 for the fake anti-virus software.
You can avoid these scams by being cautious of links in Twitter search that look like those above. More generally, it's smart to avoid downloading software from unknown sites.
The Twitter Trending Topics Scam in Pictures
Here's the full process of the scam, and screenshots of each stage supplied by Panda Security.
Step 1: Malicious Tweets and Links in Twitter Search
Step 2: Prompt to Download Flash or Other Video Software
Step 3: Virus Warnings and Prompt to Buy Anti-Virus Software