According to the Bureau of Labor Statistics, the following 15 jobs account for more than 25 percent of total U.S. employment. For comparison purposes, the largest job retail sales employs 4.21 million people, or 3.2 percent of the total American work force. By contrast, a mere 660 people are employed as prosthodontists, 1,170 as geographers and 870 as radio operators nationally.

So what are these mega-professions that employ so many people? According to the BLS, the following jobs are America's most popular:

1. Retail salespeople: Perhaps a testament to consumerism, retail salespeople rank No. 1 in job popularity. Besides great communication and customer service skills, retails salespeople also have to know how to close a deal; many of these jobs are commission-based.

Total employment: 4.21 million

Salary: $24,630*

Requirements: A high school diploma is preferred for entry-level positions. For those who hope to move on to retail management, a bachelor's degree is helpful.

2. Cashiers: Cashiers are responsible for working registers, monitoring cash drawers and taking payments at establishments that range from supermarkets and gas stations to retail stores and theme parks. Like retail salespeople, cashiers must have good customer-service and people skills, since they spend their day dealing with the public.

Total employment: 3.44 million

Salary: $19,030

Requirements: On-the-job training

3. Office clerks: Workers spend their days filling a variety of roles in an office environment, from entering data to preparing mailings. An office clerk's duties may vary daily, according to the needs of the company.

Total employment: 2.81 million

Salary: $27,700

Requirements: A high school diploma and a combination of word processing, computer and proofreading skills.

4. Combined food preparation and service workers: This category primarily encompasses those who work in fast food establishments as counter attendants and food prep workers. Duties often include taking orders and accepting payment, filling beverage cups, assembling food items and providing customer service.

Total employment: 2.69 million

Salary: $18,120

Requirements: On-the-job training. Employers look for workers with excellent customer-service skills, a neat appearance and the ability to multitask in a fast-paced environment.

5. Registered nurses: Nursing is the most common profession in the health-care field. RNs treat patients in hospitals, outpatient facilities and doctors' offices. Some provide home care to patients.

Total employment: 2.58 million

Salary: $66,530

Requirements: RNs are required to have a bachelor's degree in nursing. Many hold master's degrees or an advanced practice nursing license (APRN).

6. Waiters and waitresses: Waiters and waitresses take food orders, accept payment and provide menu information to restaurant customers.

Total employment: 2.3 million

Salary: $20,380

Requirements: There are no minimal education requirements, though many establishments prefer staff to have high school diplomas.

7. Customer service representatives: Customer service workers answer questions, provide information, fulfill customer requests and handle customer complaints. While many work in call centers, others are employed in retail stores or at banks.

Total employment: 2.19 million

Salary: $32,410

Requirements: High school diploma, on-the-job training.

8. Material movers: This broad job category encompasses laborers who literally move materials. Think truck loaders, loading dock workers and baggage handlers.

Total employment: 2.13 million

Salary: $25,290

Requirements: On-the-job training

9. Janitors: Janitors are responsible for maintaining cleanliness in places such as office buildings, museums, schools and hospitals.

Total employment: 2.1 million

Salary: $24,120

Requirements: On-the-job training, ability to perform physical work for extended periods.

10. Stock clerks and order fillers: Stock clerks and order fillers work in storage facilities, warehouses, and shipping and receiving departments, ensuring that orders are properly filled, stocked, priced and accounted for.

Total employment: 1.86 million

Salary: $23,460

Requirements: High school diploma, on-the-job training, ability to perform physically strenuous work.

11. Secretaries: Secretaries (not including medical, legal and executive secretaries) provide administrative assistance to an office in the form of answering phones, greeting visitors, ordering catering services, proofreading, entering data, scanning and faxing documents, and more.

Total employment: 1.8 million

Salary: $31,060

Requirements: High school diploma, though an increasing number hold an associate or bachelor's degree. Secretaries must also have good computer and communication skills.

12. Bookkeeping, accounting and auditing clerks: Employees in these positions serve as a corporation's record-keepers. Tasks include maintaining accounts payable and receivable, documenting and tracking budgets and preparing financial statements.

Total employment: 1.76 million

Salary: $34,750

Requirements: High school diploma, though many employers now require workers in these positions to hold an associate or bachelor's degree.

13. General managers: General managers are the top executives of establishments and organizations such as restaurants, hotels, amusement parks and sports teams. They are in charge of the business's day-to-day operations and long-term goals.

Total employment: 1.69 million

Salary: $110,550

Requirements: Most hold a bachelor's or advanced-level degree.

14. Tractor-trailer truck drivers: This category includes only those drivers who operate trucks with a capacity of more than 26,001 pounds. These drivers are responsible for the timely delivery of goods along routes that may cover multiple states.

Total employment: 1.55 million

Salary: $39,260

Requirements: A commercial driver's license is necessary for all drivers. Some states also require a training program.

15. Elementary school teachers: The category includes elementary school teachers except those who teach special education. Elementary school teachers are responsible for instructing students in kindergarten through fifth grade on a variety of subjects.

Total employment: 1.54 million

Salary: $53,150

Requirements: Bachelor's degree, teaching certification.