A heat strip is a length of resistance wire that makes heat when voltage is applied. Exactly the same as the wires in a toaster. It is very simple and inexpensive to make but it is also very inefficient. It works at any temperature outside.
A heat pump is like an air conditioner system in reverse. It sucks whatever heat is in the air outside and brings it in. It uses a compressor and coils. It is much more complicated and expensive than a heat strip but it is also much more efficient. But doesn't work very well below about 40 degrees. The bonus of a heat pump is that it will also work as an air conditioner system so less total parts.
Heat pump systems will have a heat strip in it so that when it gets real cold outside and the heat pump will no longer work, the heat strip turns on and you have heat again. Albeit at greater energy costs.