wireline perforating is perforating using a wireline unit.  Tubing Conveyed is running the gun at the end of a tubing string with a workover rig.  Tubing conveyed is used more in laterals so they can get more weight to the gun and push it along.

what exactly is a tubing string?

Tubing is the pipe in the well up which the oil or gas flows to surface.  A typical North Sea well design will have 7" tube (called casing) through the reservoir and then 4-1/2" or 5" tubing inside the casing, with a packer between the tubing and the casing just above the reservoir to isolate most of the 7" casing from the reservoir fluids.

To allow reservoir fluid from the rock into theh casing and then up the tubing, you need to blast holes in the casing with 'guns'-  lots of small shaped charges.  These gun strings can either be lowered into the well on the end of an armoured electrical wire or WIRELINE, or they can be attached to the tubing called TUBING CONVEYED PERFORATING(or TCP).  TCP is quite popular as you can get bigger guns (wireline guns have to fit inside the tubing, unless you shoot, kill the well and then run the tubing, which takes time and often carries higher risk as the reservoir is open) and on horozontal or high angle wells, you can't lower guns on wireline, so you have to push them on the end of the tubing.

Wireline perforating in many areas is now only used for reperforating: blowing more holes or newer holes in the casing in an exisitng completion, to improve production or to access new producing zones.

Basically, there is no difference in the perforating gun itself between wireline and TCP.  The differences are in the conveying method and firing mechanism.  Guns that are run through tubing can be run on endless tubing or wireline conveyed.  Casing gun carriers can be wireline conveyed or run on tubing.  Typically through production tubing guns are limited in charge size by what will fit inside the production tubing before and after perforating.  This is important because there is a huge amount of energy released on detonation and the carriers are often distorted by firing.
Wireline conveyed perforating incorporates a positioning log of some kind (typically a casing collar locator) and an electrically activated firing head with the gun assembly.  As long as the assembly will slide into the well, wireline perforating is feasible.  Multiple intervals, which can be widely separated, can be perforated in a single run using select fire technology. The overall assembly length is simply the sum of the lengths of the postioning logging device, the firing head, and the individual intervals.
TCP guns can be run in on jointed tubing or endless tubing run inside the production tubing.  The well can be empty while the assembly is run in or the assembly can include a packer to isolate the new perforations from the rest of the wellbore.  Precise positioning of TCPs on jointed tubing is time consuming as logs are run through tubing to verify positioning.  Precise positioning with endless tubing is next to impossible, but is still done if the positioning is not critical e.g. long zones or laterals.  TCP firing heads are generally either mechanical (drop bar activated) or pressure activated (triggered by pressuring up with nitrogen or fluid).  Coil or endless tubing TCPs are all pressure activated devices often a ball is pumped down to close a circulating port so the tubing can be pressured up.