Complicated means there are many strongly coupled parts that you have to understand before you can understand the whole. Computer programs tend to be complicated (one sign of a good computer program is that you can understand and work on pieces of it without having to understand the entire thing).
Abstract means that the idea itself isn't tied to a particular instance of what it is describing. Interfaces in Java are abstract. The idea of a group in mathematics is abstract. Abstraction is usually an attempt to make an idea less complicated so you can focus on its essence.
Complicated means there are many strongly coupled parts that you have to understand before you can understand the whole. Computer programs tend to be complicated (one sign of a good computer program is that you can understand and work on pieces of it without having to understand the entire thing).
Abstract means that the idea itself isn't tied to a particular instance of what it is describing. Interfaces in Java are abstract. The idea of a group in mathematics is abstract. Abstraction is usually an attempt to make an idea less complicated so you can focus on its essence.