Does a land that is also an artifact count toward affinity for artifacts?

Affinity for Artifacts reduces the cost of a spell by {1} for each artifact you control (CR 702.41a). The keyword cares only about whether a permanent is an artifact — it does not exclude artifacts that also happen to be lands.

A permanent's types are not mutually exclusive. Under CR 205.1, a permanent can have multiple card types simultaneously, such as 'artifact land.' As long as the permanent has the artifact type, it qualifies for Affinity for Artifacts, regardless of what other types it also has.

Cards like Darksteel Citadel, Great Furnace, or Seat of the Synod are explicitly artifact lands. Each of them counts toward reducing the cost of a spell with Affinity for Artifacts because they each carry the artifact type.

Example: You control two Islands, one Seat of the Synod (an artifact land), and one Ornithopter (an artifact creature). You cast Thoughtcast, which costs {4}{U} but has Affinity for Artifacts. You control 2 artifacts (Seat of the Synod and Ornithopter), so Thoughtcast costs {2}{U} instead — the Seat of the Synod reduces the cost just like any other artifact would.

Note that the land still counts toward your one-land-per-turn limit and produces mana normally; the artifact type simply adds the additional quality of reducing Affinity costs (CR 702.41a, 205.1).