Definition
A family of circuits $(C_{1}, C_{2}, \ldots)$ is uniform if some log space transducer $T$ outputs $\langle C_{n}\rangle$ where $T$'s input is $1^{n}$. (from http://en.wikipedia.org/wiki/Boolean_circuit#Uniform_Boolean_Circuits)
Can anyone exlain this? I know what boolean circuits are, so only explanation needed is what and transducer exactly are.
Uniformity definition means that there is a deterministic Turing machine running in logarithmic space $\mathsf{L}$ that generates the description of the $n$th circuit from the unary encoding of $n$.
To have a better idea of what this is talking about, assume that we have a Turing machine $M$ in $\mathsf{P}$. Then for every input size $n$ we can generate a circuit of size $poly(n)$ that compute $M$ on inputs of size $n$. So we obtain a family of circuits $\{C_n\}_{n \in\mathbb{N}}$. However these circuits are not arbitrary independent circuits but coming from a single machine $M$.
The definition is good for large classes like $\mathsf{P}$ but it doesn't work for small complexity classes. For those classes we use deterministic logarithmic time $\mathsf{DLogTime}$ in place of $\mathsf{L}$. In place of printing the whole circuit we just require it to compute any given bit of the description of the $n$th circuit in logarithmic time.
A transducer is a finite state automata that (in effect) has both an input tape and an output tape. So instead of the normal DFA/NFA computation where the automaton has an input that it recognises (or equivalently, no input at all, and an output that it generates), a transducer reads from the input and writes to the output. Otherwise the definition is in essence a DFA/NFA.
So in this case the transducer takes a number (in the form of a string of $1$s - so $4$ would be $1111$) and produces the encoding of the circuit $C_{n}$ ($\langle X \rangle$ means the encoding of $X$ under some [sensible] encoding scheme). So even though the family of circuits is infinite, they can be compactly expressed by giving the transducer.
More details about transducers can be found on the all-knowing wiki (side note, the name transducer is also used to mean some completely different things in other fields).
