Definition. Let $(\Omega,\mathscr{U},P)$ be a probability space. Let $A,B\in\mathscr{U}$ be two events with $P(B)>0$. $P(A|B)$, the probability of $A$ given $B$ is defined by
$$P(A|B)=\frac{P(A\cap B)}{P(B)}\ \mbox{if}\ P(B)>0$$
If the events $A$ and $B$ are independent,
$$P(A)=P(A|B)=\frac{P(A\cap B)}{P(B)}$$
i.e.
$$P(A\cap B)=P(A)P(B)$$
This is true under the assumption that $P(B)>0$ but we take this for the definition even if $P(B)=0$.

Definition. Two events $A$ and $B$ are independent if
$$P(A\cap B)=P(A)P(B)$$

Definition. Let $X_i:\Omega\longrightarrow\mathbb{R}^n$ be random variables, $i=1,\cdots$. Then random variables $X_1,\cdots$ are said to be independent if $\forall$ integers $k\geq 2$ and $\forall$ choices of Borel sets $B_1,\cdots,B_k\subset\mathbb{R}^n$
\begin{align*}
P(X_1\in B_1,X_2\in B_2,&\cdots,X_k\in B_k)=\\
&P(X_1\in B_1)P(X_2\in B_2)\cdots P(X_k\in B_k)
\end{align*}

Theorem. The random variables $X_1,\cdots,X_,m:\Omega\longrightarrow\mathbb{R}^n$ are independent if and only if
\begin{equation}
\label{eq:indepdistrib}
F_{X_1,\cdots,X_m}(x_1,\cdots,x_m)=F_{X_1}(x_1)\cdots F_{X_m}(x_m)
\end{equation}
$\forall x_1\in\mathbb{R}^n$, $\forall i=1,\cdots,m$. If the random variables have densities, \eqref{eq:indepdistrib} is equivalent to
$$f_{X_1,\cdots,X_m}(x_1,\cdots,x_m)=f_{X_1}(x_1)\cdots f_{X_m}(x_m)$$
$\forall x_i\in\mathbb{R}^n$, $\forall i=1,\cdots,m$, where the function $f$ are the appropriate densities.

Proof. Suppose that $X_1,\cdots,X_m$ are independent. Then
\begin{align*}
F_{X_1,\cdots,X_m}(x_1,\cdots,x_m)&=P(X_1\leq x_1,\cdots, X_m\leq x_m)\\
&=P(X_1\leq x_1)\cdots,P(X_m\leq x_m)\\
&=F_{X_1}(x_1)\cdots F_{X_m}(x_m)
\end{align*}
Let $B_1,B_2,\cdots,B_m\subset\mathbb{R}^n$ be Borel sets. Then
\begin{align*}
P(X_1\in B_1,\cdots,X_m\in B_m)&=\int_{B_1\times\cdots\times B_m}f_{X_1,\cdots,X_m}(x_1,\cdots,x_m)dx_1\cdots x_m\\
&=\left(\int_{B_1}f_{X_1}(x_1)dx_1\right)\cdots\left(\int_{B_m}f_{X_m}(x_m)dx_m\right)\\
&=P(X_1\in B_1)P(X_2\in B_2)\cdots P(X_k\in B_k)
\end{align*}
So, $X_1,\cdots,X_m$ are independent.

Theorem. If $X_1,\cdots,X_m$ are independent real-valued random variables with $E(X_i)<\infty$ ($i=1,\cdots,m$) then $E(X_1\cdots X_m)<\infty$ and
$$E(X_1\cdots X_m)=E(X_1)\cdots E(X_m)$$

Proof.
\begin{align*}
E(X_1\cdots X_m)&=\int_{\mathbb{R}^n}x_1\cdots x_m f_{X_1,\cdots,X_m}(x_1,\cdots,x_m)dx_1\cdots x_m\\
&=\left(\int_{\mathbb{R}}x_1f_{X_1}(x_1)dx_1\right)\cdots\left(\int_{\mathbb{R}}x_mf_{X_m}(x_m)dx_m\right)\\
&=E(X_1)\cdots E(X_m)
\end{align*}

Theorem. If $X_1,\cdots,X_m$ are independent real-valued variables with $V(X_i)<\infty$, $i=1,\cdots,m$ then
$$V(X_1+\cdots+X_m)=V(X_1)+\cdots+V(X_m)$$

Proof. We prove for the case when $m=2$. For general $m$ case the proof follows by induction. Let $m_1=E(X_1)$ and $m_2=E(X_2)$. Then
\begin{align*}
E(X_1+X_2)&=\int_{\Omega}(X_1+X_2)dP\\
&=\int_{\Omega}X_1dP+\int_{\Omega}X_2dP\\
&=E(X_1)+E(X_2)\\
&=m_1+m_2
\end{align*}
\begin{align*}
V(X_1+X_2)&=\int_{\Omega}(X_1+X_2-(m_1+m_2))^2dP\\
&=\int_{\Omega}(X_1-m_1)^2dP+\int_{\Omega}(X_2-m_2)^2dP\\
+2\int_{\Omega}(X_1-m_1)(X_2-m_2)dP\\
&=V(X_1)+V(X_2)+2E[(X_1-m_1)(X_2-m_2)]
\end{align*}
For $X_1,X_2$ being independent, we have $E[(X_1-m_1)(X_2-m_2)]=0$. This completes the proof.

References:

Lawrence C. Evans, An Introduction to Stochastic Differential Equations, Lecture Notes