If you work for a company that offers an SEP IRA, your employer must make the same contribution, as a percentage of salary, to your SEP IRA and SEP IRAs as any other eligible employee. The main difference between a SIMPLE IRA and an SEP IRA is that only employers can contribute to SEP IRAs, but employees can contribute to SIMPLE IRAs with their paycheck through elective deferrals. They just have different types of tax advantages, and IRAs with SEP may be more suitable for companies with several employees. Beyond that, SEP IRAs work much like traditional IRAs, in the sense that if your employer funds an SEP IRA for you, you own the account and make the investment decisions.
The main difference between an SEP IRA and a Roth IRA is that SEP IRAs offer tax-deferred growth in your investments, while Roth IRAs offer tax-free growth and withdrawals during retirement. Yes, you can contribute to both an SEP IRA and a traditional or Roth IRA (assuming you meet the income limit requirements) in the same year. Also note that you don't need to reduce your contribution to the SEP IRA to also contribute to a traditional IRA. An SEP IRA is a tax-deferred account, meaning that, as with a traditional IRA, contributions are made with pre-tax dollars and withdrawals are taxed as ordinary income.
The government imposes no restrictions on contributing to both an SEP IRA and a traditional IRA in the same year. An employer offering an SEP IRA must contribute a uniform amount, based on the percentage of salary, both to their own SEP IRA and to the SEP IRAs of each eligible employee. The only way to transfer the full non-deductible amount to a Roth IRA would be to transfer all of your traditional assets (this includes the full value of your SEP IRA and other traditional IRAs). If you are an employee covered by an SEP IRA, employer contributions don't reduce the amount you can contribute to an IRA for you, but the amount of your traditional IRA contribution that you can deduct may be reduced to certain higher income levels, due to the combination of the two plans.