These are examples of basic brake problems and usually mean something is wrong in the brake system. Left alone, any of these problems will likely only get worse, so have the brakes checked as soon as possible.

The Problem Is Caused By What To Do

The brake pedal goes to the floor Leak somewhere in the system Check/correct fluid level; Have system checked
  Brakes out of adjustment Check automatic brake adjusters

Spongy brake pedal Air in brake system Have brake system bled
  Brake fluid contaminated Have system drained, refilled and bled

The brake pedal is hard Improperly adjusted brakes Have brakes adjusted
  Worn pads or linings Check lining/pad wear
  Kinked brake lines Have defective brake line replaced
  Defective power brake booster Have booster checked
  Low engine vacuum (power brakes) Check engine vacuum

The brake pedal "fades"' under pressure (repeated hard stops will cause brake fade; brakes will return to normal when they cool down) Air in system Have brakes bled
  Incorrect brake fluid Check fluid
  Leaking master cylinder or wheel cylinders Check master cylinder and wheel cylinders for leaks
  Leaking hoses/lines Check lines for leaks

The car pulls to one side or brakes grab Incorrect tire pressure Check/correct tire pressure
  Contaminated brake linings or pads Check linings for grease if greasy, replace
  Worn brake linings Have linings replaced
  Loose or misaligned calipers Check caliper mountings
  Defective proportioning valve Have proportioning valve checked
  Front end out of alignment Have wheel alignment checked

Brakes chatter or shudder Worn linings Check lining thickness
  Drums out-of-round Have drums and linings ground
  Wobbly rotor Have rotor checked for excessive wobble
  Heat checked drums Check drums for heat checking; if necessary, replace drums

Brakes produce noise (squealing, scraping, clicking) Worn linings Check pad and lining wear
  Loose calipers Check caliper mountings
  Caliper anti-rattle springs missing Check calipers for missing parts
  Scored or glazed drums or rotors Check for glazing (light glazing can be removed with sandpaper)

Brakes drag (will not release) Incorrect brake adjustment Have brakes checked
  Parking brake stuck or adjusted too tight Check cable where it enters the brake backing plate. In winter, water frequently freezes here
  Caliper pistons seized Have calipers checked
  Defective metering valve or master cylinder Have system checked
  Broken brake return springs Check brake return springs, replace if necessary

Brake system warning light stays lit One part of dual circuit inoperative
Defective warning light switch
Differential pressure valve not centered
Have brake system checked