I am very much interested to know how images with text of different sizes and fonts are created. Can any one please suggest any tutorial to ease the process?
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As others have already pointed out, these are called "word clouds".
Word clouds are essentially visual frequency tables of words, where the font size of the word is determined by the word frequency. Thus, to create the graphic, you first need a corpus (for example, a speech made by someone, a text file of all your blog posts, a data dump of all of your emails...). The text is parsed, and the frequency of words is tabulated. Based on the frequency of the words, an algorithm will determine how big the font should be relative to the other word frequencies.
More advanced versions of the software used to make these charts will allow you to first drop "stop words" or to aggregate the counts of singular and plural versions of a word (thus, "eye" and "eyes" will be counted together).
The easiest place to get started is "wordle" (linked in Leoni's answer) or Many Eyes. These let you enter the corpus directly, or they let you enter a previously weighted list in the form of
word:frequency or even
word:frequency:hexColor. If you are into data science and are comfortable with programming, you can also look at the open source R programming language and the "wordcloud" package.
While a bit of a craze, word clouds also have some shortcomings. The biggest one for me is that this suffers from the same problem that many choropleth maps suffer from. The algorithm to determine the relative font size of the words does that only based on the frequency. It doesn't take into consideration the length of the word. So, in the example you've given, "innovation" and "original" seem to be the same font size, but the area covered by these is different since "innovation" is a longer word.
You can generate that kind of graphic at http://www.wordle.net/.
The answer above is correct but if you really wanna do this manually, it requires practice and creativity. I mean, it would be easier if you draw it on paper first and trace it on Photoshop or Illustrator like this one http://www.skillshare.com/classes/design/Digitizing-Hand-Lettering-From-Sketch-to-Vector-Lettering-II/214693817 but if you wanna do it manually with just fonts and sizes and colors, you'd have to practice extensively. I myself have to admit that I'm not good at it either.