What's the difference between inside IR35 and outside IR35?

Being 'outside IR35' means your engagement is genuinely treated as self-employment through your company, so you're taxed on your company's profits via Corporation Tax and can extract income via salary and dividends as normal. Being 'inside IR35' means the engagement is treated as employment for tax purposes, so Income Tax and National Insurance are deducted at source (usually by the fee-payer) before the money reaches your company, similar to being on payroll. Status is assessed contract by contract, not for your career as a whole, so you can be inside IR35 on one engagement and outside on another.