[kevinpet]

HFT is about making money by supplying liquidity or making money by exploiting the portions of the system which are designed to supply liquidity. You don't want to end up with a position. You want to net out the day with no position in any stocks. HFT is done by an HFT fund which is usually trading off credit extended by an agressive hedge fund who they split the profits with.

Algos are about getting a trade done at the least cost. You want to buy 100M shares for fund XYZ or sell 10M shares for fund ABC. Algos are run by brokers on behalf of their buy-side clients. A simple algo is something like TWAP, the time weighted average price, which attempts to buy a large position over the course of the day gradually. The purpose of this algo is to avoid moving the market. This is suitable for the "build up a large position because I'm bullish on this industry" type of trade.

Another algo might be IQx, a package offered by CovergEx which tries to quickly execute the trade by sending it out to multiple dark pools and the major exchanges at gradually worse prices until the whole order fills. (Or at least that's my reading of the marketing material.)