Hi
I think you have read alot as the base concepts can't be explained in detail. Since you have raised here, a gist of those concepts for your reference.
Employee Engagement - Employee engagement activities develop a sense of connection between the employee and the employer, in turn reducing turnover and assisting in the retention and development of talent. It basically refers to all events or festival celebration, training, R&R programs, internal social networking sites, intranets, surveys, suggestions campaigns, etc. All activities that enable the employer to engage with the employee and vice versa...
Employee Relations - Employee relations is the HR function responsible for maintaining rapport between employees and the employer. This may include settling disputes between an employee, manager, workgroup, department, or other employee. The employee relations representative may act as a mediator in the dispute or interpret laws and rules governing the dispute and try to resolve the dispute to the satisfaction of both parties. Employee relations also deals with disciplinary issues, terminations, conflict and dispute resolution, absenteeism, harassment, communication policies, health issues, and workplace safety issues.