In the following we describe some basic concepts that are fundamental for understanding (marine) climate and climate variability. We begin with a brief historical review on earlier and more modern concepts of climate and subsequently define marine weather and marine climate as used throughout the remainder of this book (Section 1.2). In Section 1.3 an overview of the present understanding of the global climate system is provided. In particular, the components of the climate system and the general circulations of the atmosphere and the oceans are addressed. We further show that the planetary-scale features of atmospheric circulation may be determined without any knowledge on regional details. In Section 1.4 concepts for understanding observed climate variability are discussed. Here emphasis is put on internally driven climate variability, in particular on the concept of stochastic climate models. This concept is considered to be fundamental for understanding observed climate variability ranging from several months to hundreds of years. We conclude with a discussion of the interplay between large-scale1 climate and regional-scale climate. It is shown that there are some large-scale constraints that may be used to describe regional climate variations in terms of large-scale changes. Also, feedbacks of the regional on the large scale are discussed. It is demonstrated that in general the details of regional climate are unimportant, while its statistics indeed do matter for realistically modeling the observed climate.