Gameplay mechanics

The gameplay and physics of the Sonic games had changes over time, but some game details have remained the same.

The main purpose, as expected from Action-Adventure games, is to reach the Goal of the Zone or Stage. To do this Sonic the Hedgehog, or depending on the game another selected hero like Miles "Tails" Prower or Knuckles the Echidna must overcome the levels environment, obsticles, traps and enemies to get to the other side of the level.

Throughout these games there will be oppotunities to earn points and also to enter the Special Stage where a Chaos emerald can be earned. However of later games the concept of the Special Zone have been dropped to the side-scrolling Sonic games.

Counters
Counters can be seen throughout the levels during gameplay keeping score of lives, rings and time.

Time
The maximum time limit of each stage is 100 minutes. However the purpose of a timer in the games isn't to show how much time you have left, but how long you have played the level.

As Sonic is about speed, the clock is to encourage speedplay and records as usually the Goal rewards quickest time.

Rings
Rings are one of the common gameplay items in almost every Sonic game. They are mostly used for protection from attacks and damage. 50 rings can grant access to the Special Stage by Warp Rings or Star Post.

Rings are also in Special Stages as collecting them can be neccisary to earn a Chaos Emerald (or at other times orbs.)

Lives
Lives are how many times you can play a certain level over. Lives are lost when Sonic (Or whoevever hero the player chooses in the game) is hurt without rings, or crushed or fallen below the level.

There are ways to increase your lives. The two most major methods throughout the series is for the hero to break open a "1-up monitor" these rare item boxes can be found in difficult places. Another method is to collect 100 rings which result in an extra life. Lives can also be rewarded as bonuses after completing an Act or level.

At the bottom left corner of the screen during play, you'll see an icon of the heroes face which is same to the Life Monitors. This icon will tell you how many lives you have. The default ammount to begin with is 3 and if you loose a life, you continue from the last Checkpoint you left off.

Points

 * Wikipedia article: Score (gaming)

Points are earned from defeating enemies which cost 100 points. Points are tallied at the end of the act or stage along with rings and time converted into points which depending on how much may reward you with bonus items. (And in later games a rank)

Checkpoint

 * Wikipedia article: Checkpoints

Checkpoints are marked areas of the Zone or Stage in which if you loose a life, you can return to this point after defeat.

Lamppost
First appearing in Sonic the Hedgehog, they didn't really resemble Street lights. More simply posts with spherical tops which spin and flash red once Sonic touches them.

Star Post
These checkpoints have more purpose. When passed with 50 rings portals to the Special Stage will open above them.

Meters and Gauges
The game does not rely on a "life bar" since Sonic or his friends will loose a life just under 1 hit. However, there lives can be sustained by possessing rings or have a barrier to protect them.

Meters which register power or health have been used in more recently in Sonic games. The earliest being Tails Adventure where Tails' duration of flight is kept. Tails strength to fly increases everytime he gets a Chaos Emerald.

Meters for powers have also been used such as in Sonic Heroes where when the Team Blast meter is filled, the whole team can perform a powerful attack. An Action Gauge has been used in Sonic the Hedgehog (2006) where characters abilities are limited to this gauge, the meter however could recharge over time or needed Light Cores to recharge.

Boosting
Sonic Rush and it's sequal utilized a meter for boosting. A meter for boosting has been developed further for the game Sonic Unleashed where rings give Sonic the ability to break into instant and powerful speed. This very meter used again in Sonic Colors which is charged by the Wisps Hyper-go-on energy. (The meter in the NDS variation of Sonic Colors mostly resemble the boost meter from Sonic Rush while the Wii meter resembles the boost meter from Sonic Unleashed.)

Health
Some enemies in the series have health meters of there own. First sene during boss battles in Sonic Adventure and some tough enemies in Sonic Heroes and Shadow the Hedgehog have there own life meter to let the player know how much health they have remaining.

The first playable character to actually use a life bar would be Tails and Dr. Eggman during Sonic Adventure 2. The mechs they drive had meters that register the power of there machines.

Sonic the Werehog is the first Sonic character to fully utilizes a health meter which replenishes health by rings. Sonic also has an Unleashed Gauge which fills when he destroys enemies. When filled he can unleash his powers and doubles the damage dealt and halves the damage taken (in the latter's case, it makes Sonic invincible instead in the PS3 and X360 version of Sonic Unleashed).