Normal Yield Curve
When long-term interest rates are higher compared to short-term interest rates, the shape of the yield curve is upward sloping.
Steep Yield Curve
This curve is normally observed at the beginning of an economic expansion or just at the end of a recession. The slope of the yield curve increases as the difference between long-term yields and short-term yields become wider. The inherent assumption behind such a curve could be that while short-term economic conditions warrant lower rates, factors like inflation, etc. could rise in the medium / long-term justifying much higher long-term rates.
Flat Yield Curve
When there is no change in market outlook on interest rates, we get flat yield curve. This is because yields are almost same across tenors.
Inverted Yield Curve
When short-term interest rates are higher than long-term interest rates the shape of yield curve takes downward sloping. This happens when markets expect high volatility in the near future however long term story remains same.