I have struggled with social anxiety for as long as I can remember.
I have spent so much of my waking life dealing with this problem... I know it inside out, it's part of who I am. I try not to let it define me but I think it's always going to be there in the background.
Speaking as a heterosexual male, it's crippling for relationships with the other sex. I'm sure it's hard for male/females and any sexual orientation but from where I'm standing as a heterosexual male it's nearly impossible to establish an intimate relationship... even casting aside notions of a serious relationship, a one night stand to share desires is almost as impossible... the rough and tough blokes with all the arrogant charisma step in waaaaay before the socially anxious guy in the corner. Sorry to talk about it as just raw meaningless sex... I'm illustrating an extreme is all. The socially anxious girls? they are so often are swept up by the charisma of the expert "player"... and suffer a different fate after having their needs met (many get stuck in dysfunctional relationships for example)... I'm not sure which is better, I suppose the grass is always greener on the other side... all in all social anxiety just messes things up all around.
Alcohol... oh alcohol how you have been the saviour of a completely isolated existence... but oh how you are a double edged sword and ruin things in the long run. Going to a party the goal was to drink a few drinks very very quickly to begin with... then some social interaction was possible... but it was always dysfunctional so it was either a matter of making an excuse and leaving or becoming ridiculously drunk so I didn't feel the anxiety. Either way waking up the next day was always a depressing experience, every party was survived... every single one... wow, that is so **** depressing when I think about it like that, I don't think I have ever genuinely enjoyed the process of going to a social function, it's always been a carefully engineered process of survival.
Graded exposure helps... together with CBT (cognitive behavioural therapy)... really pulling apart your experiences and thoughts and testing them to see how valid they really are. For example describing and rating your expected anxiety and experience while going to a local cafe and sitting down by yourself and having a coffee and something to eat (which is ****ing hard if you have social anxiety)... then actually going through the experience and coming back and analysing the ACTUAL experience, finding that one makes it out to be much much more daunting than the actual experience, so you can begin to challenge your own thoughts and perceptions.... going through that process and practising really does help.
How can I explain social anxiety? Sitting at home again on a Friday night, for the ????th week in a row... wanting so much to be involved in something, but not able to go to the corner shop to get milk for tea... facing the social experience of interacting with the shop keeper is much too daunting... how the hell can I go out? Let alone tell any girl that I think she's attractive!!! Ha ha, you've got to be kidding me. Order home delivery food and tip very well the person who delivers it to make sure it's a super positive happy interaction with no risk at all of confrontation... another Friday night.
I meet people on YouTube and some forums (like this one) that understand social anxiety. The internet has been a great medium for sharing insights with other people that have social anxiety.
I think it get easier with age, it has for me anyway

I could never have written about these things on an internet forum a few years ago... partially because I couldn't articulate it, but as much because I was so fearful of the judgement of others on the internet. I guess I feel a bit safer here too because people seem to be a bit more understanding than the average corner of the internet.