This was the official language of the 1st and 2nd IPC in 1998 and 2000 respectively. It separated the model of the planning problem in two major parts: (1) domain description and (2) the related problem description. Such a division of the model allows for an intuitive separation of those elements, which are (1) present in every specific problem of the problem-domain (these elements are contained in the domain-description), and those elements, which (2) determine the specific planning-problem (these elements are contained in the problem-description). Thus several problem-descriptions may be connected to the same domain-description (just like several instances may exist of a class in OOP (Object Oriented Programming) or in OWL (Ontology Web Language) for example). Thus a domain and a connecting problem description forms the PDDL-model of a planning-problem, and eventually this is the input of a planner (usually domain-independent AI planner) software, which aims to solve the given planning-problem via some appropriate planning algorithm. The output of the planner is not specified by PDDL, but it is usually a fully or partially ordered plan (a sequence of actions, some of which may be executed even in parallel sometimes). Now lets take a look at the contents of a PDDL1.2 domain and problem description in general...
(1) The domain description consisted of a domain-name definition, definition of requirements (to declare those model-elements to the planner which the PDDL-model is actually using), definition of object-type hierarchy (just like a class-hierarchy in OOP), definition of constant objects (which are present in every problem in the domain), definition of predicates (templates for logical facts), and also the definition of possible actions (operator-schemas with parameters, which should be grounded/instantiated during execution). Actions had parameters (variables that may be instantiated with objects), preconditions and effects. The effects of actions could be also conditional (when-effects).
(2) The problem description consisted of a problem-name definition, the definition of the related domain-name, the definition of all the possible objects (atoms in the logical universe), initial conditions (the initial state of the planning environment, a conjunction of true/false facts), and the definition of goal-states (a logical expression over facts that should be true/false in a goal-state of the planning environment). Thus eventually PDDL1.2 captured the "physics" of a deterministic single-agent discrete fully accessible planning environment.