All protected Wi-Fi networks are secured using some version of Wi-Fi Protected Access (WPA/2) which rely on the 4-way handshake defined in the 802.11i amendment. Key Re installation AttaCK (KRACK) abuses design or implementation flaws in the WPA2 cryptographic protocols. KRACK works by targeting these 4-way handshakes and thus both WPA and WPA2-certified products are affected by this Krack Attack. 
WPA (sometimes referred to as the draft IEEE 802.11i standard) became available in 2003. To put things in perspective this issue has always been present for the last 14 years, but has recently gained quite a bit of attention. Let's first understand what this WPA2 vulnerability is and asses it's impact.