Formal probation means that you will have an assigned probation officer to whom you must report and follow his/her instructions (seek employment, not drink alcohol, not leave county, etc.).
Jail alternatives may include work furlough, home confinement, etc. The cost varies. This is something you should really discuss with the attorney that is representing you (retained or public defender).
You have left out some needed information.
I have never heard of 18 months of formal probation for either a misdemeanor or felony.
In part, what you describe sounds like you may have pled to a simple possession for personal use. You were directed to attend a drug treatment course/program.
This is called a deferred entry of judgment. If you complete the course and stay out of trouble for a minimum of 18 mos. to a max of 36 mos. the case is dismissed.
I STRONGLY suggest that you contact your attorney and go over your case disposition.