Can a DSL or cable wire between my modem and the ISP be manipulated in any way at the hardware level to sniff the traffic data?
In theory, yes - the data isn't encrypted and the line code isn't a secret either.
In practice, that isn't too easy however. You could use off-the-shelf parts to terminate the DSL line, bridge to Ethernet and then create another DSL line towards the ISP (classic MitM). The Ethernet link in the middle can be easily tapped into and data can even be manipulated.
Alternatively, you could build custom hardware to tap into the line directly. Due to the non-trivial line codes the DSL variants use that would require quite sophisticated circuitry. This path can only provide readability, no manipulation.
The technical situation for cable modems (DOCSIS) is somewhat similar. In contrast to DSL, DOCSIS already provides provisions for encryption - how hard that encryption actually is depends on your ISP and can vary significantly (56-bit DES to 128-bit AES, albeit with only 1024-bit RSA). Without considerable background information, I wouldn't depend on that encryption.
In real life, neither method is very affordable. It's likely easier to try to break Wi-Fi encryption or even break into the house and tap an Ethernet cable.
what information could they get and how could i prevent it?
Everything that isn't properly encrypted. Make sure you encrypt all your sensitive data - use HTTPS, SSL/TLS, IPsec, ... - and protect your gear from being tampered with (to prevent an unauthorized root CA certificate from being installed).
Of course, all that is valid in respect to third parties - anyone who needs to physically tap your connection. Someone who's already within the loop - your ISP, your governmental authorities (depending on country) or similar don't have to put up with physical limitations but can much more easily tap into any flow.