Same as ipa, the deb is another extension of an executable. To install it, for example you can open ifunbox and on the left and select Install Cydia App in the top menu select the Copy option from the pc, you select the deb you have. Now just restart your phone. I particularly what I do is with ifunbox leave it in a folder, for example:
/ private / var / mobile / Documents
and the iFile program in the phone (down from cydia) I select and give to install