The sponsor is the entity that sets up the scheme, usually this is the employer. The sponsor's role includes defining membership requirements and rules, benefits, contribution levels, investment choices and (in most cases) it will also make contributions. The sponsor will usually choose to outsource some of these tasks (e.g. outsourcing the investment solutions to an investment manager). In a defined benefit scheme the sponsor is also financially liable for the solvency of the scheme, and must aim to ensure that the assets are always high enough to cover the liabilities.
The provider is the company which actually administers the scheme. In the UK this will usually be a big life insurance company such as L&G, Aviva, Prudential, Standard Life, etc.
The employer should be fairly obvious.
The trustee is responsible for running the scheme in line with both the law and the trust deeds and rules. They must do so in the best interests of the members (i.e. not the employer) and act impartially. Trustees are usually a mixture of people who are chosen by the sponsor and people who are voted in by the members. If anything goes wrong in the scheme, trustees may be personally liable, and so they will often seek professional advice (mainly from actuaries!) when making decisions.