In his introduction to Implicit and Explicit Learning of Languages Nick C. Ellis describes implicit and explicit learning in the following terms:
Implicit learning is acquisition of knowledge about the underlying structure of a complex stimulus environment by a process which takes place naturally, simply and without conscious operations. Explicit learning is a more conscious operation where the individual makes and tests hypotheses in a search for structure. Knowledge attainment can thus take place implicitly (a nonconscious and automatic abstraction of the structural nature of the material arrived at from experience of instances), explicitly through selective learning (the learner searching for information and building then testing hypotheses), or, because we can communicate using language, explicitly via given rules (assimilation of a rule following explicit instruction). (Ellis 1994b: 1 f.)
In the definition, implicit learning is conceived as a natural, simple and conscious learning process whereas explicit learning is described as a process which includes conscious operations such as the making and testing of hypotheses.