  Coronal loops are the building blocks of the X-ray bright solar corona. They
owe their brightness to the dense confined plasma, and this review focuses on
loops mostly as structures confining plasma. After a brief historical overview,
the review is divided into two separate but not independent sections: the first
illustrates the observational framework, the second reviews the theoretical
knowledge. Quiescent loops and their confined plasma are considered, and
therefore topics such as loop oscillations and flaring loops (except for
non-solar ones which provide information on stellar loops) are not specifically
addressed here. The observational section discusses loop classification and
populations, and then describes the morphology of coronal loops, its
relationship with the magnetic field, and the concept of loops as
multi-stranded structures. The following part of this section is devoted to the
characteristics of the loop plasma, and of its thermal structure in particular,
according to the classification into hot, warm and cool loops. Then, temporal
analyses of loops and the observations of plasma dynamics and flows are
illustrated. In the modeling section starts some basics of loop physics are
provided, supplying some fundamental scaling laws and timescales, a useful tool
for consultation. The concept of loop modeling is introduced, and models are
distinguished between those treating loops as monolithic and static, and those
resolving loops into thin and dynamic strands. Then more specific discussions
address modeling the loop fine structure, and the plasma flowing along the
loops. Special attention is devoted to the question of loop heating, with
separate discussion of wave (AC) and impulsive (DC) heating. Finally, a brief
discussion about stellar X-ray emitting structures related to coronal loops is
included and followed by conclusions and open questions.
