Which respirator filter do I need?
The CDC has a technical bit about the regulations for respirators (in the US, at least). There are two types of respirators - particulate and chemical / gaseous / vapor (they also make combination).
Particulate respirators are the more important for the average home user, and are regulated by the National Institute for Occupational Safety and Health (NIOSH). They fall into three main categories, depending on how resistant to oil they are: N, R, or P. Filters also have an efficiency rating, which is based on how many particles they are able to remove from the air. These are 95%, 99%, or 100% (or really, 99.97%). Put the two ratings together, and you have P100, N95, and more.
From the CDC link, they provide a summary for selecting a particulate filter:

If no oil particles are present in the work environment, use a filter of any series (i.e., N-, R-, or P-series).
If oil particles (e.g., lubricants, cutting fluids, glycerine, etc.) are present, use an R- or P-series filter. Note: N-series filters cannot be used if oil particles are present.
If oil particles are present and the filter is to be used for more than one work shift, use only a P-series filter.
Selection of filter efficiency (i.e., 95%, 99%, or 99.97%) depends on how much filter leakage can be accepted. Higher filter efficiency means lower filter leakage.
The choice of facepiece depends on the level of protection needed--that is, the assigned protection factor (APF) needed.

TH2P seems to be related to some kind of power / active filtering. HEPA is another standard, based on efficiency of removing particles. If you have allergies, you may want to look for a HEPA filter, but any of the standard NRP 95/99/100 would be suitable for you. Given that you likely will not have contact with oils, an N-rated filter would be sufficient. Get the N100 unless it's a lot more expensive than the N99 or N95.
Gas / vapor filters are color-coded by the types of vapors they filter, but that's beyond the scope of this answer.