National Electrical Code (Article 250.20 section A) defines "Low Voltage" as it applies to home distribution systems as less than 50 volts.
(A) AC Systems of Less Than 50 V. Alternating-current systems that
operate at less than 50 V are not required to be grounded unless:
(1) The primary exceeds 150 V to ground
(2) The primary is ungrounded
This includes systems such as cable television, network and telephone, doorbell, etc. Note that standard telephone wiring can typically "ring" at 48 volts and still provide an electric shock.
The reason the code makes the distinction for low voltage is so that some types of wiring can be exempt from additional safety requirements such as conduit usage, breakers, installation method, etc. Even so, there are still violations where installers confuse the issue (Top ten violations in low voltage systems PDF).
There are (to my knowledge) no current limits for data lines, except for the rating of the cable itself. Since most UTP network and telco cables use 24 or 26 gauge wire, this would theoretically be around 3.5 amperes, but you have to consider length, temperature, etc. to properly determine current load (discussion at allaboutcircuits).
Guessing that you might be wanting to use data lines for higher power applications, I thought I'd add some additional commentary. If you are wanting to use Cat5 for power-over-ethernet applications, you'll probably want to stay under 1A or less per conductor. This is because of the voltage drop and resistance on longer distances. Keep in mind that higher voltages are more suitable for transmission (longer distances) as long as you stay under the 50V "low voltage" specification. Even though charts show that 24AWG can carry 3.5A, I wouldn't consider this safe unless working with a very short cable. Power-over-ethernet standards are confusing, but generally they are 48 volts or less at 600mA or less.
Electrical characteristics (at Wikipedia) for UTP Cat5E show a max current of 0.577A per conductor and a max voltage of 125V.