Characteristics

PostScript is described as a

dynamically typed, concatenative programming language

What does this mean?

Dynamically Typed

A type is a property associated with a certain programming constructs that can be used to determine what operations are legal.

Dynamically typed means that the type, i.e. the property that determines legal operations, is only known at runtime.

Concatenative

A concatenative language is part of programming paradigm that favors composition of functions over function application. This might be as illuminative as a broken flashlight.

What is meant by that can be illustrated by a code example in a pseudo language. Function application, i.e. applying functions to their arguments, typically looks like

x = f(w)
y = g(x)
z = h(z)

Here function f is applied to argument w, that result is applied to function g, etc.

In a concatenative language the same example would typically look like

w f g h

It's probably best to try your hand on an actual concatenative language to get a feel for the trade-offs between the different paradigms. Luckily PostScript provides just such a language.