Qjackctl is a GUI frontend for JACK that allows you to access and manage many of JACK's features. It allows you to choose your interface, settings, make connections, etc. It is very popular and is also the most fully featured front end for JACK.
The Advanced Linux Sound Architecture is a audio framework, which is part of the Linux kernel. Both JACK and Pulse Audio use ALSA as their backend. Unless you are using a firewire device (see FFADO), it is ALSA that you will be looking at to see if your audio interface is supported. ALSA also has MIDI device support. A list of ALSA supported audio interfaces can be found on the ALSA website.
The LV2 plugin standard is the successor to LADSPA. LV2, which stands for LADSPA version 2, is intended to address the limitations of LADSPA. LV2 plugins can easily be extended and most LV2 plugins come with custom GUI's. Supported hosts include Ardour, Qtractor and Carla. A popular collection of LV2 plugins is the Calf plugin suite.