An activity tracker, also known as a fitness band, specifically measures fitness metrics. Yes, it may tell the time as well, but it is designed to measure movement, heart rate, and calories.
A smartwatch is designed to pair with a smartphone and has many other integrated features and apps beyond fitness. It receives texts and emails. It notifies you of phone calls and integrates with Bluetooth technology.
A technology builds, higher-end watches and activity trackers merge, becoming more similar with each generation. Even as the two mesh together, there are differences in durability with smartphones still being vulnerable to extreme athletic activities.