The japanese sencha is a steemed tea. Depending on how long the fresh leafs are out of steem, there are 3 general types of sencha. Light steemed (Asamushi): 30 seconds, medium steemed (Chumushi): 60 sec., deep steemed (Fukamushi): 90 - 120 sec. It's very rare to see strong deep steemed sencha (Kyomushi). 

These three types has different consistence, color and taste.

Asamushi is yellow and clear liquid. The taste is stronger, but with light aroma, poor after taste and slide a bit sweeter than the other types.  It has very good refreshing effect. 

The deep steemed (Fukamushi) has a deep green color. Clarity is cloudy, mild astragrency and also one of the best characteristics - grassy taste. 

The medium steemed sencha is something in the middle. 

Curious fact: There are only few websites selling original japanese green tea.

Unfortunately even some japanese companies buy garbage from other countries and sell it to Europe and other continents to look like it is produced in Japan. They make easy money selling huge quantities for shops and sushi restorants. So, my advice is to buy sencha from these few online shops. The original sencha is in small packages: 50 g., 100 g. and rarely 200 g. 

If you look bigger package, run!

Here are the sites I know:

www.yuuki-cha.com
www.o-cha.com
www.zencha.net
www.hibiki-an.com

I'm sure there is another good sites, but it's enough for me and my 10 years experience.

Next: How to prepare sencha? Why people compare japanese green tea with the wine?

Enjoy