You can buy a new Echo unit. The Echo is just a brand name for batteries that are in the "eGo" class of e-cig batteries. The battery is itself the entire device, including a power switch to activate whatever juice delivery device you put on top of it. Once that battery dies out, the entire thing needs to be replaced.
Ego class of battery.
More advanced PV's known as "mods" are essentially a more durable battery holder utilizing replaceable batteries, with a power switch and in some cases additional control buttons to change the voltage or other features. These work similar in function to a flashlight holder. In fact that is how "mods" got their name as the first ones were modified from existing items for use in the e-cig industry.
Mods are more heavily constructed to make them much more durable than the eGo batteries. The batteries they use typically last longer; up to 1 year or longer. Those batteries also are much cheaper to replace and are more powerful than an eGo battery. Many people find it easier to carry a single mod with them and perhaps a small spare battery to last an entire day as opposed to using the smaller eGo batteries where one would need to carry 2-3 entire units to last an entire day's worth of vaping.
Being that they are also larger in size than an eGo, they have a larger variety of juice delivery devices available and with much larger juice capacities that also can go 1-2 days without juice refills.

Provari mod with replaceable batteries.