In reality, there is a lot of overlap between the two. HR stands for 'human resources' and, broadly speaking, relates to all things which apply to employees or workers such as, recruitment, training, pay and benefits and more straightforward workplace issues. These might typically be referred to as the "softer" elements of employment.
Employment law is practised by solicitors and is the legal framework made up of numerous laws, rules and regulations which apply to the employment relationship to support HR, manage issues and to stipulate the legal rights and responsibilities of both employers and employees which must be complied with. This requires a more specialist and in depth knowledge of the numerous laws which apply to different situations and the ability to ensure that steps taken by an employer do not breach the laws.