From what I believe, ISA's are just high interest savings accounts. What are 401Ks?
ISA's aren't just a high interest savings account - they're far more than that
Whilst they offer "high" interest (in quotes as at the moment nothing really offers high interest!) the neat part is... the interest is
tax free. That's forever - until you remove it.
Every year you have an allowance - it's currently £15,000, which you can put into an ISA. Basically tax sheltered savings. If you put 15k per year in, then after 10 years you'd have 150,000. However, due to interest you may actually have 180,000 or something... and each year the interest generated would be ~ 3%, which is £5,000... which isn't taxed (usually taxed at 40% I think?). So the next year your ISA would be £185,000, and the year after £190,300...etc.
Furthermore, ISA's aren't just about cash savings - mine are held in a stocks and shares ISA. The risks are greater (you may get back less than you invested) but the rewards
can be greater too (18 months after investing £25,000 - it's now worth £29,000). Furthermore, dividends for high dividend paying companies can far outstrip interest
and dividends within an ISA are
tax free again. Right now, mine is currently reinvesting dividends back into shares, which means my 'portfolio' is growing nicely (hence the actual increase in investment). The best performing fund last year hit 18.9% interest effectively (that's with dividends reinvested) and is by Neil Woodford.
You'll hear in the news about ISA allowance around March time - that's because the financial year is April and you can invest £15,000 each year between April/March. I invested all the money by using £10,900 in March 2013 and £14,900 in June 2014 (two different financial years, despite only being a few months apart).
ISA's aren't for short term though - it's a long term investment. You get a £15,000 allowance per year, but if you took the money out you can't "put it back" again. You'd have to use your allowance instead. However, you can transfer from one ISA to another without issue (if it was paying too little interest and better options are available).
You don't need much - you can put as little as you like in. Some are treated like savings accounts and will deposit £25 per month into your ISA. The important part is
starting the investment, not the amount you can put into it!