An unlocked phone is a phone that is not tied to any specific carrier, while a universal phone is not tied to any specific network.
In many cases, phones are both unlocked and universal.
Smartphones are designed to work on either GSM or CDMA phone networks — think AM versus FM radio. Here in the US for example, AT&T and T-Mobile are GSM networks while Sprint and Verizon use CDMA. A universal phone can be used on any of those networks.For years, unlocked phones were almost exclusively GSM. But recently, more smartphones are universal, like the iPhone X, Google Pixel 2 and Moro G6 When in doubt, always check with the phone manufacturer or your carrier for compatibility.