Generic tax question on capital gains.

[CobraGoat]

How do US capital gains taxes affect ones ROI in a stock?

I buy X amount of shares of US stock Y as an individual.

a. I sell stock within that year for an ROI of 11%. What are the tax implications?

b. I hold the stock beyond the year i bought it but during that year it shows an ROI of 11%. Any tax implications for that year?

Additionally, how do US taxes affect mutual fund turnover within a given year? How does that affect the funds performance?

I apologize for my ignorance. If these questions are too vague or general I will specify. If they are nonsensical, please contribute anything helpful or insulting.