Video 8 and Hi 8 , are analogue recording formats
Hi-8 has a higher resolution
Digital 8 is digital, essentially using the DV codec you get on minDV tapes onto the tape format of Hi-8
The actual tape for all 3 are near identical but for Hi-8 and D8 better grade tape from Video 8 is specified
Hi- 8 camcorders can play video 8 recorded tapes but Video 8 cannot play Hi 8 tapes
Many early Digital 8 camcorders were developed to allow digital recording ( like MiniDV) onto Hi-8 tapes
They could also play Hi 8 and video 8 recordings and infact convert them to digital for firewire to the PC
The better ones are broadly similar to MiniDV apart from size but the last ones were manufactured a few years ago.. and they were mainly Sony only
Because of error correction with digital , you are more likely not to lose video with minor tape problems but all tape based formats suffer from dirty heads, and tape being thrashed by faulty mechanisms
If you are new to camcorder use I would suggest you give these older formats a big miss and wide berth
They would mainly be of interest to anyone who already has a collection of tapes and need the backward compatibility
Even if keeping cost down was a consideration , most aent likely to be new and potential repairs , parts ect might make getting them now an unwise move
The very least you should consider is a MiniDV tape camcorder
In fact even miniDV is not nearly as popular as it used to be what with HDD ans flash card now being the media of choice
Have a read
http://en.wikipedia.org/wiki/8_mm_video_format