symmetric monoidal (∞,1)-category of spectra
Universal algebra is the study of algebraic theories and their models or algebras. Whereas abstract algebra studies groups, rings, modules and so on — that is, models of particular theories — universal algebra is about algebraic or equational theories in general.
Traditionally, the subject studies models of algebraic theories in the category of sets. The category-theoretic approach abstracts the traditional notions, to study models in more general categories. There are two ways of doing this: by using monads and by using Lawvere theories.
As with the category-theoretic understanding of many other branches of mathematics, the advantage of doing things this way is not so much the obtaining of new results as the unification of many previously disparate points of view. Examples might include how a Hopf algebra is the same thing as a model in a category of vector spaces of the theory of groups, or how computational side-effects in the theory of programming languages may be understood in terms of free algebras?.
PRO,
Martin Hyland and John Power, The category theoretic understanding of universal algebra: Lawvere theories and monads (pdf).
Anthony Voutas, The basic theory of monads and their connection to universal algebra (pdf)
Fred Linton, An outline of functorial semantics, LNM 80 (TAC Reprints).
Max Kelly and John Power, Adjunctions whose counits are coequalizers, and presentations of finitary enriched monads, JPAA 89, 1993.