A writing retreat is like a working vacation: the ideal version is to take a trip or sneak off to some quiet spot where one can work without distractions. Most writers love them; as a rookie pro I was a bit bewildered by the whole concept. Go somewhere to write? Why do you have to leave home? Then I was invited on my first retreat with another author and spent two days at the beach doing nothing but writing and talking about writing and swapping chapters and reading and doing more writing. We only left our hotels rooms to sit by the pool and sun ourselves while we proofed pages and discussed story issues. I have to admit, it was a little like spending 48 hours in writer heaven.
The idea of going on a writing retreat to some vacation-type spot is wonderful, too but the cost of transportation, lodging, meals and so forth can make it an expensive proposition. Fortunately there are other, less costly ways for writers to take a retreat, like a free writer's residency. This is when some writing or arts organization provides you with lodging and sometimes other amenities so you can write. Generally you have to apply for a residency, and if you get it also cover the cost of travel and personal expenses, but the free accommodations and no-distractions environment may be worth it (and if you're interested in finding a residency, check out the online database of opportunities at Poets & Writers magazine here.
An even cheaper type of writing retreat is the virtual version. Get together with a writing buddy and set a goal for a day, and update each other on your progress via an Internet connection (Skype, Twitter, Facebook, etc.) If you have a favorite chat room you can have a word war (challenge each other to write as much as possible in a short amount of time and post your counts as you work in the chat room.) If you'd rather go solo, visit an online typewriter site to do some distraction-free work (Big Huge Labs has one here) or try out one of the document creation/editing/storage sites like Google Docs or Zoho which offer free personal accounts.
I take mini-retreats all the time and never leave my house -- my back porch happens to be a quiet, comfortable spot for me to edit pages, and when I'm working out there the family knows to give me some space. Once a year my guy goes on vacation with the kids and leaves me home alone, too; that becomes my week to having a working vacation and write whenever and wherever I like. If you can work out something like that with your family I definitely recommend it.
In April and July this year the wonderful folks over at National Novel Writing Month are holding two Camp NaNoWriMo online writing retreats, during which you can choose to write 50K or set your own writing goal from 10K to 999.9K, share a virtual "cabin" with other writers based on your preset preferences as to genre, age, word-count goal, and desired activity level, and work on the project of your choice (novel, script, short story, epic poem -- you decide.) These retreats are free to participants; if you have a log in from NaNoWriMo you can use that to sign up and join in.
Remember that a writing retreat isn't just a thing you do, it's also a state of mind. You choose to spend x-amount of time to focused entirely on the work. If you can't do that at home, find a place congenial to you where you can. That can be a bookstore cafe, the quiet room at your local library or a picnic table at the park. Pack a lunch, grab your laptop and head out (and if you're going to an outdoor location, check the weather forecast first.) If you have a friend with a spare bedroom, you might ask if you borrow it for a day. Test drive different places and see where you're most productive, too.