Hi! 
I would like to know something about the history of grafitti, modern
grafitti like bombs and walls in our time. What different types of
grafitti is called. Thanks:)

Hello, Irma!

This was an interesting question to research. I've assembled a number
of links that will provide you with quite a bit of reading matter on
this subject.

You'll find an excellent and highly detailed history of graffiti here:

New York City Cyber Bench
http://www.at149st.com/history.html

Some comprehensive histories of graffiti:

Davey D's Hip Hop Corner
http://www.daveyd.com/historyofgraf.html

Hip Hop Network
http://www.hiphop-network.com/articles/graffitiarticles/historyofgraff-arpone.asp

Graffiti Removal Program
http://www.graffitiremovalpdx.org/aboutgrafhist.html

Here's a discussion of "taggers" and gang graffiti:

National Alliance of Gang Investigators Association
http://www.nagia.org/graffiti.htm

A glossary of terms related to graffiti and tagging:

Archived from Washington State Narcotics Investigators Association
http://216.239.57.100/search?q=cache:OTid5pXKJb8C:www.wsnia.org/graffiti_update.htm

Regarding graffiti on bombs, here's a close-up of the tail fin of
"Little Boy," the atomic bomb which was dropped on Nagasaki. Notice
the rather chilling "Here's to You!"

US National Archives and Records Administration
http://media.nara.gov/media/images/29/13/29-1248a.gif

Here is an article about anti-Arab graffiti on bombs used during
Operation Desert Fox:

Southnews
http://southmovement.alphalink.com.au/southnews/981223-ADC.htm

Controversy over an anti-gay slur on a bomb destined for Afghanistan:

Total Obscurity
http://www.totalobscurity.com/mind/wtc/antigay-bomb.html

Search terms used:

"history of graffiti"
"bomb graffiti"

I hope this is useful. If anything above is unclear or incomplete,
please request clarification before rating my answer; I'll be glad to
offer further assistance.

Best wishes,
pinkfreud

Thank you so much, this site is just great! you have done more then
enough, in spite of my blurry question:)