I need to know what the abbreviations mean, e.g 'VBD', 'NN', etc. I am learning text processing and now I want to get verbs in the sentences, so that I need to know which abbreviations represent any form of verbs.
No one answered the question, then I will answer it myself. Thanks to @user12075 for the link. I didn't find it when I was googling it.
From the above link, I know that nltk uses The Penn Treebank's POS tags.
nltk.help.upenn_tagset() will give you the list.