The LaTeX picture environment is a special environment for inserting simple drawing commands into a LaTeX file. This environment was obviously not intented to replace more sophisticated graphical formats such as Postscript or PDF (and their associated design tools: Adobe Illustrator, Macromedia Freehand,...), but was merely aimed at easily incorporating small and simple drawings (e.g. graphs, electrical schematics,...)

Yet picture commands are flexible enough to allow one to build rather complicated drawings, including circles, rectangles, lines and vectors, bezier splines.

Besides, using epic/eepic or PsTricks packages largely increases the set of available commands, and amongst other things, allows for shape-filling (and colours with PsTricks) and a better rendering of shapes.

Finally, one of the main benefits of using the picture environment is the ability to incorporate text containing any LaTeX command allowed in LR mode, e.g. equations, arrays or user-defined commands.

...
\unitlength 1mm
% an easy way to re-scale the whole picture :
\linethickness 0.15mm
% starts environment and set picture size :
\begin{picture}(100,100)
% a vector starting from (0,0) with slope (1,0) and length 50mm :
\put(0,0){\vector(1,0){50}}
% same as above, but horizontal.
\put(0,0){\vector(0,1){50}}
% a line with slope=4.
\put(10,20){\line(1,4){5}}
% an equation surrounded by a rectangular box :
\put(20,30){\framebox(10,10){$\frac{x}{y}$}}
% same as above, but w/o the surrounding box.
\put(55,00){\makebox(0,0){$\vec{i}$}}
% a dashed box
\put(60,5){\dashbox{5}(10,20){$\vec{j}$}}
\put(0,0){\makebox(0,0)[tr]{0}}
% a circle, with center (30,30) and diameter=10mm
\put(30,30){\circle{10}}
% a disk
\put(10,30){\circle*{5}}
% a bezier spline
\qbezier(10,10)(20,0)(30,20)
\end{picture}
...