I'm not aware that there is any + for CDs; they are either CD-R or CD-RW.
DVDs come in + and - versions, and I'm sure other contributers will be able to tell you what the differences are if you need to know.
Huawei P10 review
1995-2015: How technology has changed the world in 20 years
An overview: What leading creative agencies are doing to improve diversity
New iPad, iPhone SE & Red iPhone 7 on sale now
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.