Hi Islam,
SAP DMS is an SAP instance which is used to store sale invoices,documents etc. For SAP DMS two types of storage can be used: 1) MaxDB, 2) FileSystem Storage.
SAP DMS can be installed from SAP Netweaver DVD in which it is named as SAP Content Server. You can download the Content Server DVD and Max DB from Service Marketplace.
There is no additional License Cost for SAP Content Server (DMS).
For more details you can download the SAP Content Server Installation Guide from Marketplace (S-user ID is required):
https://websmp201.sap-ag.de/~sapidb/011000358700003351232006E.PDF
Once you have installed the SAP Content Server, it is integrated with ERP System.
Main T-codes used are:
CS_ADMIN - To Administer Content Server from ERP System
OAC0 - Create repositories.
This is just introduction.
If you have any doubts, let me know.
Thanks,
Gaurav