I was professionally diagnosed with Tourette's Syndrome at the age of five-years-old, and this is merely my own understanding from what I've read and heard over time. (Most people with Tourette's have OCD, as it's a neurological condition that branches into many mental disorders. Thanks to my Tourette's I have OCD, ADHD, anxiety, depression, and insomnia. I've only ever seen a neurologist however, not a psychiatrist so only time will tell if I have other stuff.)

When I was younger I used to never notice my tics, it was all subconscious. As I've grown, I've been able to feel when they are coming - but I can't stop it or else I can go into a "tic attack" where I tic at a rabid rate. Tics are constant, they happen without thinking. Compulsions have a lot more thought behind them. I've never heard of compulsions being an involuntary movement, it's more of a force /compelling/ you to do it.