Key 401K People

• Participant - person in the plan; considered to be active if received contributions during the year
• Plan sponsor - employer who sets up the plan
• Plan Administrator - responsible for running the plan
• duties include: appointing legal, filing returns, reviewing, directing, etc. 
• typically the employer unless otherwise appointed/hired by the employer
• Third Party Administrator - an entity/organization/firm hired by the employer to assist in actual Plan Administrator
• duties include: designing, producing, amending, and restating plan documents; assist in processing transactions, testing the plan, etc.
• Trustee - someone who has exclusive authority and discretion to manage & control Plan assets
• duties include: making decisions regarding investments and managing the assets 
• Custodian - the entity that has physical control of contents of the plan
• duties include: protecting assets from theft and keeping it secure

Reference: IRS Definitions for Employee Benefit Plans