Varieties of Marxism?

I find from a link that Richard Singer posted and some related discussions  that there may be several varieties of Marxism. I have been following only one thread that I read in Robert Heilbroners's "The Worldly Philosophers". Even there I could not understand the thesis, antithesis stuff. It is about capital's emphasis on profits rather than useful production, the corresponding need for low wages, unemployment and the problems that might arise due to too much accumulation of capital. The period with which we are familiar with from the first world war to 1970 ort so seems to be an exceptional period and perhaps should not be taken as a model. The link which Richard posted Marx blogged to death.
P.S. Link to Heilbroner chapter on Marx