Iwasawa worked with so-called mathbb{Z}_p-extensions: infinite extensions of a number field F with Galois group Gamma isomorphic to the additive group of p-adic integers for some prime p. Every closed subgroup of Gamma is of the form Gamma^{p^n} , so by Galois theory, a mathbb{Z}_p -extension F_infty/F is the same thing as a tower of fields F = F_0 subset F_1 subset F_2 subset ldots subset F_infty such that extrm{Gal}(F_n/F)cong mathbb{Z}/p^nmathbb{Z}. Iwasawa studied classical Galois modules over F_n by asking questions about the structure of modules over F_infty.
More generally, Iwasawa theory asks questions about the structure of Galois modules over extensions with Galois group a p-adic Lie group.