half life is the amount of time for half of the amount used to be flushed out of system.
Example: you take Dianabol 50 mg pill and it has half life of 1-3 hours. It means that after 1-3 hours you're only having 25 mg left of it. That's why active life is twice as long.
other compounds are active for as long as the half life as much as you can see. but there are also those that tend to slowly build up in the system to fully kick in.
there's just too many details and pharmacology here. Simply use in recommended frequency the specific compound you're interested and you'll be good.
PS: There's also detection time (in case of a test) which is very different too.