There is some basic fundamental difference between DVD-ROM and DVD-R. Here it is.
Once you buy DVD-ROM, you can not write anything in it and also you can not delete anything from it. The only way to remove data from DVD-ROM is to destroy DVD. The space used in DVD-ROM is not necessarily full (4.7GB). The data in DVD-ROM may be as small as 0 bit.
DVD-R is the DVD in which you are capable of writing data, music or movie or anything else through your PC by using DVD Writer (Hardware).
Blank DVD available in market is DVD-R, you can insert data in it as many time as you want until its all segment is burned or before writing (burning) select the option to finalize it.
DVD-R becomes DVD-ROM once before writing any data in DVD-R you do not select “multisession option” (in Nero) or similar (for other than Nero software for DVD Writing) and burn it. If you do not select “Multisession” option before writing (burning) DVD, the Writing software will assume that you do not want to add any further data in DVD, and it will finalize the DVD in the process of burning and will close DVD for any further data burning. In this situation DVD-R, it will become DVD-ROM.
If DVD-R is open for data addition, i.e if previous data added using multisession option, you can hide the previously added data be removing the index of previously burned data from the software and burn the DVD. Actually you are not removing the data from DVD, but by doing above, the laser of DVD reader will not be able to reach at sector of DVD in which previously data were burdened. So it will be similar to previous data of DVD is deleted.
In DVD-R you can write only on the sector of DVD which is still unburned. In blank DVD, DVD writer start writing data on DVD from Center, you can see the color change in the segment in circular shape which get burned by data. Once a segment is burned by data, this segment can not be burned again by another data. In DVD-RW you are able to rewrite on same segment of DVD-RW many times.
While burning data with multisession option you should not remove the content of the DVD from the the content list of DVD which is visible in DVD writing software if you want the DVD reader should read the previously burned data as well.