As you reach various population levels, you'll unlock corresponding township levels, buildings, and bonuses, as indicated below.

Little Hamlet (500)
Elementary School, Medical Clinic, Landfill Site, Taxes, Loans, Garbage, Healthcare, Education.

Worthy Village (1000)
Police Station, Fire House, 2km x 2km area, Districts, Policies, Second Loan, Industry Specializations, Services Policies, Fire Department, Police Department, Unique Buildings, Forestry Specialization, Agriculture Specialization.

Tiny Town (1500)
High School, Decoration, Level 2 Unique Buildings, Pet Ban, Smoking Ban, Parks, Recreation.

Boom Town (2600)
Bus Depot, Advanced Wind, Turbine, Cemetery, 2km x 2km area, Transport, Level 3 Unique Buildings, Bus, Recycle, Recreational Use, Ore Industry Specialization.

Busy Town (5500)
City Planning Policies, Level 4 Unique Buildings, Free Public Transport, Heavy Traffic Ban, Oil Industry Specialization.

Big Town (7500)
Metro Station, University, Incineration Plant, 2km x 2km area, Taxation Policies, Level 5 Unique Buildings, Metro, Education Boost, Tax Raise, Tax Relief, High Density Residential Zone, High Density Commercial Zone, Office Zone.

Small City (11,000)
Train Station, Cargo Train Terminal, Hydro Power Plant, Level 6 Unique Buildings, Train, Small Business Enthusiast, Big Business Benefactor, Industrial Space Planning, High Tech Housing, Highrise Ban.

Grand City (20,000)
Harbor, Cargo Harbor, Third Loan.

Capital City (32,000)
Nuclear Power Plant, 2km x 2km area, Ship.

Colossal City (44,000)
Nuclear Power Plant building, 2km x 2km area.

Metropolis (65,000)
Airport, Airplanes, 2km x 2km area.

Megapolis (80,000)
2km x 2km area, Monuments.