Graphics -

This is known to be one of the best games of all time for graphics, the details on the cars and tracks are amazing, and there are no glitches that I have found in any of them.
The different views you can have in the car along with the brilliant realistic graphics make it feel like you're actually driving for real.

Arcade Mode -

Single Race - One standard race as to where you can choose how many laps, the weather, the cars etc.
Time Trial - This is where you are able to set a time with your best lap and try and beat it with every lap you complete. You are able to choose any car on any of the tracks available for this.
Drift trial - The test in this is how far you can drift the car on any of the tracks available.
Split Screen - Play against your friends with the 2p split screen

GT Mode -

Where you are able to start your career and are able to race other people online with the cars you have purchased and modified.

Online - This has a wide selection of races available as to where your modified car can be put up against other cars that have been done up by other players, You are also able to trade cars online with other players.

Special events -
These are events you can play to earn cash and are good fun to play, here is a list of them all -
GT Karting experience
Nascar School
Top Gear Test Track
AMG Driving academy
GT Rally
Grand Tour
Seb Loeb's Rally Challenge
Each of these events get unlocked as you go through your career, the higher the level, the more you can play and the more cash you are able to play for.

A spec - Go through different races with different types of cars to complete each section and get a reward for doing it. Here are the sections -
Beginner/Amateur/Pro/Expert/Extreme
These all have a selection of races and one big race (usually best of 5 races) to finish that series.
You will have to buy all kinds of cars to play the different races from beetles to Ferrari's.

B Spec - This is where you are able to create a driver and manage his career, it is similar to a spec in the fact that has all the same races and levels and cash to buy cars with etc. However you are not the one driving the car you only manage the driver and can tell him what to do at a certain stage of any race. For example if he is way ahead in a race you are able to tell him to slow down and play it safe to reduce the risk of crashing, however if he is behind the field coming towards the end of a race you are able to tell him to speed up and do all he can to overtake. The challenges get harder as you go through the different series and you have to time your moves correct in the later stages if you are going to win in the harder races.

Cars

There are more cars available in this game than any other racing game and the detail in each of them is incredible, you can get cars like go karts all the way to cars like the zonda. There are 51 car makes to choose from and a selection of cars in each one.

Another good addition to GT5 is the used car dealership, this makes it easy to find a half decent used car if you do not have much money in the career, it is also good if you need a certain type of car for a race in a series because you can get it cheap and will generally only use it for one race.

Licences

There are 6 different licences to achieve on the game and each of them has a harder challenge to complete. The licences are -
B/A/IC/IB/IA/S
There are a selection of challenges you must complete in each one to be able to pass it, each of them have a certain limited time or distance as to where you can get gold, silver or bronze medals to pass, all you need is to get a bronze on each to pass however if you get silver or gold you will unlock better cars at the end of them.
Some examples of the challenges are - Stopping challenge, Steering, Braking into corner, overtaking, slalom etc.

Tracks

Just like the cars there are many tracks to choose from in the game than any other. I think that a very good edition is the Top Gear test track to the game; this is now one of the most popular tracks in the UK and has been made famous by the TV show.

They have original tracks like deep forest raceway and trial mountain circuit etc.
They have world tracks like Daytona, Monza, Laguna Seca etc.
They have city tracks like Rome circuit, special stage route 5 etc.
They have snow/dirt tracks like Chamonix, Eiger Nordwand etc.

Overall

Overall I feel this is one of the best car games I have ever played with incredible detail, great range of cars and great online play.