What is "web seed"?
According to the wikipedia page:
Web seeding was implemented in 2006 as the ability of BitTorrent clients to download torrent pieces from an HTTP source in addition to the swarm. The advantage of this feature is that a website may distribute a torrent for a particular file or batch of files and make those files available for download from that same web server; this can simplify long-term seeding and load balancing through the use of existing, cheap, web hosting setups. In theory, this would make using BitTorrent almost as easy for a web publisher as creating a direct HTTP download. In addition, it would allow the "web seed" to be disabled if the swarm becomes too popular while still allowing the file to be readily available.
As for the safety aspect, I would presume it is of no higher risk than normal torrent.
While normal seeds are other peer to peer users, a webseed is hosted on a server. Webseeds are used to guarantee the availability of the download, even if no other p2p users are sharing it.
[Web Seeds are the] location on the web for any files within the torrent. If you add such locations, then peers with ability to use webseeding can download from these sources.
When you author a torrent you can add any locations where you have uploaded the contents, or existing web locations. The source must be identical files unless there are webseed links for all the included files, you may not be able to download 100% of the file from webseed because of boundary data that's spread across files. Using the align file boundary option will take care of that shortfalling, but will result in creation of padding files, which are harmless, but [which] some find objectionable.
Source: this Comet Forums post
What seeds give the torrent client the ability to download torrent pieces/data from an http source in addition to the swarm. So if you have a file somewhere on the internet, you can simply add its link to your torrent. Now if the swarm is weak, the torrent client will fetch data from the http source. The advantage, of course, is that a publisher can create a torrent of a file which is already hosted on his server and not worry about seeding it full time, while the user can obtain the data directly from the http source or through the torrent. Either way, the user will get the data from the http source. However, if the torrent becomes popular and self-sustainable, the torrent client will fetch data from the swarm and only use the http seed for pieces which are not available or are deficient in the swarm.
This is a very good approach towards file distribution which can be used by artists, producers who distribute their content online. Using web seeds, they can keep their torrents alive for as long as their servers are up. A perfect balance between load-balancing and content availability.
Source: post by Keshav Khera