People often think these terms are interchangeable but that is not true. All reclamations are polders, but polders are not always land reclamations. Land reclamations can be divided into (unqualified) reclamations and dykings. A rule of thumb:

Polders consist of land whose (natural) groundwater is regulated artificially and can therefore deviate (substantially) from that of its immediate surroundings. Polders are generally encompassed by a dyke, but they do not always lie below sea level (e.g., the  Beekbergen polder at Apeldoorn, which is 10 m above sea level). A remarkable polder is that of the former island of Schokland in that its groundwater is kept at a level higher than that of the surrounding Noordoostpolder.

Land reclamations are polders that are created by draining pools or lakes. Most of the reclamations are enclosed by a ring canal and a ring-dyke. They are generally – but not necessarily – below sea level. (e.g., the Beemster polder, 3 to 4 m below sea level).

Dykings are reclamations that have developed by closing off a (shallow) part of the sea, an inland sea or a tidal inlet (sea polders) or a part of a river bed (river polders) by means of a protective dyke (e.g., the Koegras polder at Den Helder, approximately at sea level).

The 20th century IJsselmeer polders can be considered to be a hybrid form of reclamation and dyking (e.g., the Noordoostpolder, 2.5 to 4.5 m. below sea level).