Is the a standard which I can use for our contacts? I want to build an application which hosts our clients contact information. Is there a standard I should use so it is possible to sync with as many clients as possible?
In Outlook I can find something about "LDAP" can you give any tutorials maybe?
I would want to support 2 way syncing so that I can edit something in outlook for example and it uploads it to our servers. Like with calendars the CalDAV
Check out Microformats. I'm not sure if they're supported in the places you want, but they're an interesting little bit of bootstrapped markup. They have a contact info format called hCard.
CardDAV is the emerging standard. It's like CalDAV, but stores the contact resources in vCard files and makes them accessible using extensions to WebDAV. So far, only Apple have implemented it on the server with Mac OS X Server 10.6's Address Book Server, and on the client with Address Book on Mac OS X 10.6. iPhone Support is still lacking (as of firmware 3.1), and there are still several limitations with Apple's implementation of Address Book server, namely no Organisation-wide contacts (like public folders), and no delegation/permissions can be set on the accounts. There is also no web interface for viewing/editing them, so unless your clients are all on 10.6, they're left in the cold.
An alternative approach would be to use Exchange 2007 and EWS (Exchange web services) to store and access centralized contacts, that will sync back to clients.