Hi~!
Two different standards by different groups of manufactures were developed called "+" and "-".
The "+" formats include DVD+R and DVD+RW discs. The "-" formats include DVD-R and DVD-RW discs.
At first DVD burners supported either "+" or "-" but not both. Most drives today support both standards.

There are many different disc formats but for data there is either DVD-ROM or DVD-ROM (UDF). Also, like CD dics DVD-ROM discs can be multisession.

DVD-ROM no multisession - you can write to the disc only once with DVD+R and DVD-R dics. With DVD+RW and DVD-RW discs you have to erase the whole disc before your write to it again.

DVD-ROM with multisession - the disc is not closed after you write data to it. You can open the session again with a program like Nero and add data to it. However, you also have the option at any time to close or finalize the disc thus preventing any more data being written to it. You can erase data but the space cannot be recovered. Like "DVD-ROM no multisession" DVD+RW and DVD-RW discs can be used again by erasing them.

DVD-ROM (UDF) uses something called Packet-Writing to make a DVD+RW or DVD-RW disc act like a hard drive. You can add and delete files at will. You can use Windows Explorer to add and delete files on the DVD drive with drag-and-drop. Also, with this format space is recovered on the DVD disc when files are deleted. UDF can be used on DVD+R and DVD-R discs but erasing files does not recover any space. Before Windows XP you had to install a special software to read discs with UDF format. There is some native support for UDF in Windows XP but it was not until Windows Vista that there was full support for it.

Once when i tried to burn data on a dvd-r with Nero(multi-session), it erased and overlapped the previously written content. What went wrong ?

Hello
Which DVD players can play +/- DVDR's?
Please tell me clearly.
Thanks.