Tactical shirts are specialized upper under-armor attire made to protect from cold and moisture while allowing ideal mobility and combat readiness. Modern tactical shirts are also very comfortable.
Yes. Unlike tactical shirts that are worn under your armor, there are ballistic shirts that fall under armored clothing where bulletproof fabric is installed inside the shirt itself.
The best army shirt will depend on your mission and location. Generally, shirts that will keep you dry and regulate your temperature well are preferred in most combat situations.