In the 24th century, a normal Federation transporter system, that sent its signal through normal space, was limited to a range of about 40,000 kilometers. The range of a subspace transporter was several light years. The matter that was transported needed to be in a state of quantum flux, which was highly unstable. It was possible to attempt to stop a subspace transporter beam-out with another transporter system tied into subspace field coils. The other transporter had to maintain pattern lock on the matter that was about to be beamed away with the the subspace transporter. Deflector shields could not stop a subspace transport.
Under normal circumstances, a subspace transport could not be detected with sensors. However, if the signature of a subspace transporter was known, it was possible to detect when a particular subspace carrier wave was approaching and focusing. This made it possible to determine where the source and target coordinates of the transporter beam were. A person who had used a subspace transporter would leave this distinct subspace signature on anything they came in contact with. This contact needed to be more than brief though. Depending on the type of transporter, the subspace signature might decay faster than normal.
Standard Federation transporters could be modified to transport via subspace by aligning the field coils tied into the system and synchronizing the phase dampers, after which the transport pattern was modulated. The transport sequence itself was longer than normal because the pattern needed to be shunted through the subspace field coils. (TNG: "Bloodlines")
Some time before 2370, the Federation had researched subspace transporter technology, but deemed such devices impractical. Development was abandoned because of the unreliability and energy requirements of the technology. The only people known to have utilized a subspace transporter were DaiMon Bok and the crew of USS Enterprise-D.
In 2370, Bok subspace transported himself two times, two Ferengi probes and Jason Vigo instantaneously between a distance of approximately 300 billion kilometers (0.03 light years) from his ship. For comparison, it would have taken twenty minutes at warp 9 for a starship to bridge the distance. Geordi La Forge and Data modified the transporter of the Enterprise-D to subspace transport Jean-Luc Picard to Bok's ship. (TNG: "Bloodlines")
In the earlier season 6 episode TNG: "Realm of Fear", Reginald Barclay cited his Transporter Theory class in which Dr. Olafson talked about "the body being converted into billions of kiloquads of data, zipping through subspace." This would seem to suggest that all transporters operate by transporting matter through subspace and would seem to conflict with the description of the difference between a normal and a subspace transporter from the season 7 episode "Bloodlines".