Do residents of mainland China need visas to visit Hong Kong?

Yes they need to obtain a visit pass when entering hongkong, but the procedures are rather simple and straight forward.
oh yea it costs a mere 20CNY.

yes and they give limited days my wife only got 4 days once and 9 the other times

Not a visa but a permit, and there are many kinds depending on the nature of the visit: tourism, training, settlement, business, etc.

Mainland residents who wish to visit their relatives in HK are required to obtain an Exit-entry Permit for Travelling to and from HK and Macau with an "endorsement for visiting relatives (Tanqing)" from the relevant Public Security Bureau Office.

Mainland residents from Guangdong Province and 28 other cities, including Shanghai, Beijing, Nanjing, Suzhou, Wuxi, Hangzhou, Ningbo, Taizhou, Fuzhou, Xiamen, Quanzhou, Tianjin, Chongqing, Chengdu, Jinan, Shenyang, Dalian, Nanchang, Changsha, Nanning, Haikou, Guiyang, Kunming, Shijiazhuang, Zhengzhou, Changchun, Hefei and Wuhan who wish to come to HK for sightseeing purpose in individual capacity are required to obtain an Exit-entry Permit for travelling to and from HK and Macau and an "endorsement for individual visit (Geren Luyou)" valid for single or double-journey from the relevant Public Security Bureau Office.

It can be complicated. So check this website for a complete detailer on entry requirements.

Not a visa, all they need is a permit