## 1 Introduction
A microgrid is generally defined as an interconnection of various DERs (intermittent and dispatchable distributed generation, distributed storage, demand response, etc.) and loads, with suitable control and electrical protection functions to enable it to operate either autonomously or in parallel with the electric grid while meeting the operational requirements in either mode. It is usually considered to be connected to the low or medium-voltage distribution network and has the desirable characteristic that it appears to the grid as a single controllable entity, thus considerably enhancing the grid integration of several heterogeneous DERs [10]. A microgrid may be designed to serve a variety of needs, but a primary goal is often the enhancement of (local) electric power supply reliability, with the microgrid's capability to operate either in parallel with the grid or autonomously [11]. A Virtual Power Plant (VPP) is yet another concept for the aggregation of a variety of DERs, of differing characteristics and technologies, operated collectively based on an advanced ICT infrastructure and an advanced EMS, to realize more benefits from the coordinated utilization of the available resources than would be possible in the case of individual resource operation [12]. While a microgrid is mainly designed to improve the supply reliability and power quality for a local load or a part of the distribution network [4] the development of the VPP concept has mainly been driven by the intention to effectively integrate DERs into various sections of the electricity market [8]. Additionally, the resources making up a microgrid (to a large extent) tend to be situated in one geographical location, whereas a VPP may aggregate resources spanning a wide geographical topography, relying on advanced ICT infrastructure and an efficient EMS to coordinate the operation of the various resources [13]. These characteristics of the VPP provide an opportunity for it to offer various system-wide services necessary for the reliable operation of the electric power supply system. Distributed generation, demand-side management, and demand response are components that have been identified as having significant potential in supporting the realization of the electric power system's operational objectives [14]. Together, distributed generation and demand response can be referred to as demand-side resources. The effective management, control, automation, and optimization of these demand-side resources in active distribution systems are the focus of this review. The work is structured thus: Section 2 investigates major differences between traditional passive distribution systems and active distribution systems. It further focuses on planning, design, and operation in these systems. Section 3 presents a detailed overview of distributed energy resources, including their classifications, and investigates the benefits of distributed generation. In Section 4, the implementation of communication technologies for control and automation capabilities in ADS, their roles in active distribution systems, vulnerabilities, and security threats associated with them are discussed. Energy storage in demand-side resources of ADNs is presented in Section 5, and the state-of-the-art control and automation strategies in distribution networks are presented in Section 6. Section 7 details a survey of various optimization methods employed on the demand-side resources of ADN while Section 8 concludes the paper and presents recommendations for further studies.