It's expected to be the biggest IPO since Facebook raised $16 billion in 2013. Bloomberg says analysts give Alibaba a $153 billion valuation.
This will have big ramifications for Yahoo, which currently holds a 24% stake in Alibaba.
Yahoo has been treated as a tracking stock for Alibaba for years.
Once the stock is public, Yahoo will be able to sell shares and pocket lots of cash.
The downside to this is that Yahoo's stock had been riding high since there was no other way to invest in Alibaba. Now that it's going to be public, investors won't have to pile into Yahoo.