It is SEO hack a.k.a Hacklink. A few days ago I wrote a detaield post about it but it has been deleted due to T&C violations

Basically, your website is vulnerable due to poorly scripted code or using outdated plugins. This guys have automatic tools and they are scanning and attacking hundreds of thousands URL's in one go. It happens completely automatically.
If you are using bespoke script then contact your developer and ask for security audit. If you are using wordpress, joomla, presta etc, make sure that all of the installed plugins (even those disabled) are updated. If you host multiple websites under one account e.g. /public_html/website1com, /public_html/website2com, /public_html/website3com and one of them is vulnerable to hacks, then all of your websites are sharing same faith as attacker can use rootshell/remote upload to gain access to your entire /public_html folder.
Go trought your directories and check for strange files/folders and also look for files that have been recently modified. If you can find affected files and they are not the part of core script files, then delete them. If they are core files, edit and remove malicious code. When you are sure that all infected files have been deleted/updated, run an update on all plugins, themes etc and change your passwors. If you host multiple domains under one account, make sure to change mysql and ftp passwords. When you done, ask your hosting provider to run a scann on your account.
If they modified files by "touch" you will not be able to track these changes as this will not update the dates. In order to track infected files, you will have to check them all. Manually check each file and those that are not a part of core engine you can delete. Everything else must be edited and malicious code must be removed. When you are sure that all infected files have been deleted/updated, run an update on all plugins, themes etc and change your passwors. If you host multiple domains under one account, make sure to change mysql and ftp passwords. When you done, ask your hosting provider to run a scann on your account.
If you don't know how to solve it, ask your host for backup or upload it on your own (hope you have one

). Then run an update on all plugins, themes etc and change your passwors.
I hope google will see it as an obvious hack and not hit mywebsite
Harvest indexed spam pages using following footprints:
site:mywebsitedotcom "Viagra"
site:mywebsitedotcom "Cheap"
site:mywebsitedotcom "Generic"
site:mywebsitedotcom "Pharmacy"
etc.
Remove unwanted pages using GWT
